All you need really (for a classically oriented tune) is a basic understanding of keys and maybe chords and chord progressions.

Basically one way to write/analyze music is to first determine the key of the music. Your solo looks like it wants to be in D major, judging by the F#,C#, and occasional Bnaturals. However, some parts flirt with being in harmonic D minor (F natural, C#, B flat). The reason it "doesn't sound quite right" is because of the inconsistency and mixing of the two keys. You're 'allowed' to modulate between D major and harmonic D minor, but you have to be consistent in which parts of which phrases are which key. I would recommend adding some F naturals OR B naturals to passages that don't sound quite right. Notice that the last 3 measures are in a consistent harmonic D minor and that they sound perfectly fine.

In writing other pieces, pick a key and stick to it. Remember that even though they're called "accidentals," they're not accidental at all. Accidentals mark a foreign chord or key modulation that is consistent across all the parts (if you're writing for more than one part).

One other random thing, powers of 2 aren't just for computers and calculators. Baroque-era music commonly used phrases of 4,8,or 16 measures for a reason--it sounds 'complete.' Try seeing if you can divide your pieces into phrases-- musical sentences--that have a start, middle, and end (the end typically being the tonic note, in the case of your solo it's D). And it's easiest to do it in powers of 2.

If you wanna get fancier (and this is important for writing pieces with multiple voices) i can talk about chords and chord progressions if you like

Sure.

So, let's say you want to write something in D major. There are two ways to approach music writing: the first being outlining the chords to fit the melody, and the other fitting the melody to the chords. Both ways are fine. I'll start with what I favor, which is writing the chords first.

So in a major scale, there are 7 notes, hence there are 7 chords you can use. The convention for naming these chords: roman numerals I-VII, uppercase means major, lower case means minor. So in order, they are
I ii iii IV V vi vii^o
The seventh chord is diminished, don't worry about what that means, I'll get into it later. But try playing each chords' triad (aka arpeggio). A major triad/chord is major because the 2nd note is 4 half-steps higher than the root, or first note (this distance, or interval is called a major third). A minor third is a distance of 3 half steps. A major triad is built out of (with respect to the root note) a root, a major third, and a fifth.
I: D F# A D (sounds major)
ii: E G B E (minor)
iii: F# A C# F (minor)
IV: G B D G
V: A C# E A
vi: B D F# B
vii^o C# E G C# (notice this one sounds different- it's root, minor third, diminished fifth)

So let's write a phrase to match a I chord in D major. The basic 'rules' are that the first and last note of the chord duration must be "in" the chord, or one of the 3 notes that make up the triad. If we took the first 4 notes of your solo, D A B C# and made it D A B D that would 'fit' in the I chord structure. (As a side note, DABC# does fit into a chord, which is the I7 chord or major seventh, but we'll get into that later)

Try playing D A B D in unison with any other measure that follows the same rules (first, last note part of chord I, other notes part of key) and you'll notice it sounds surprisingly good.

Now that you can write a melody line to a chord, we'll move onto chord progressions. Chord progressions are just patterns of chords put together. One of the most common is I-IV-V-I. You can write any of your own progressions, though some flow more smoothly than others.

So, try writing two lines that, for one measure each follow the chord progression I IV V I. The second line can just be whole notes if you want, but you'll notice immediately that there is now a much stronger structure and resonance to the piece.

Now, matching chords to a prewritten melody can be a bit more difficult just because there are a lot of other types of chords and stuff that you (and I) don't know about. But we can try, and once we've got the chord progression from the melody, we can begin to write other lines that follow the same progression. Often pieces are developed using both methods; chord-first offering a stronger structure, and melody-first offering more variations and freedom.

Anyway let's take Twinkle Twinkle and figure out the chord progression.
D D A A B B A
G G F# F# E E D
A A G G F# F# E x2
D D A A B B A
G G F# F# E E D
So let's see... It is in D major judging by the ending on D and the F#. The first line looks to be a I chord. Then..IV chord (G and D both lying in the G-B-D triad)..Next..V chord (A, E lying in the A-C#-E triad) for 2 lines. Then back to I and IV for the end. Notice that matching a chord to a melody is quite flexible. We could've said that only the G-G in the last line is a IV chord and the rest is a I chord. Either way works with what information we do have. Try writing another line that follows the first progression and matching it with Twinkle. Then try using a different chord progression and see how the sound changes.

Once you get the hang of writing with chord progressions, try making up chord progressions of your own. Each chord doesn't have to last the same amount of time as the others, and mix up minor and major chords from the 7 options you have available. Or just mess around with all the variations one set can give you. Half of popular music these days only uses 4 different ones; those being I,IV,V,vi (there was another thread about that somewhere).

Note that there are also much more than these 7 chords, there are chords for the 3 variations of minor scales if you wanna write in minor, augmented, diminished, and about 50000 types of seventh chords as well as something called neopolitan.

There is no "right" or "wrong" way to compose music...The theory teaches you how to express yourself, it is not a creative force in and of itself.

This is definitely true. Don't worry too much about all the rules I just spouted. The rules are meant to be broken. They are only guideposts to set you on your way

Unfortunately, I do not know much about the musical notation, because I use DAW's that are grid-based and am more visual, but one thing I notice is that for more complex music, where there is more than one track, a lot of people have troubles getting the bassline key right. I'll use one of your song Squidgetx, as example:



What I am talking about is, for example, at the 0:06 mark in this video, one bass note is off-key. In some cases, this can destroy the song melody completely. In this case it isn't too bad, however, but I often saw happy hardcore and trance remixes of popular songs on youtube that sounded good until the bassline became off-key. Things need to fit together. It's all about experiementing and trial and error, though.

Also, if you got for a multi-track song, with beat, basslines, melody, choir pads, for example, you need to do a minimum of equalizing or balancing, so that 1) Sounds don't get drownded out by others and you can barely hear the beat, for example. Or the opposite can happen, where everything is recorded low except one sound channel, so the user has his volume up, then all of a sudden, R.I.P. headphones.

Also, don't use a potato to record your music. If you can record directly into your computer, it's even better, although some mics are quite good too.

One thing to remember, however, is that music making is often trial and error. You may need to try different instruments or melodies before you are satisfied with your work. Also using real instruments requires practice, so at first it's best to practice with existing songs (easier ones first). You won't play Steve Vai solos on your first attempt at playing guitar. Also, you need to learn a bit about the structure of certain music genres if you are going to make music similar to that. For example, when I made trance, I listened to some popular european artists first to get familiar with the genre. You don't have to stick to mainstream genres, though, since music, after all, is a form of inspiration and freedom.

Also, don't use a potato to record your music. If you can record directly into your computer, it's even better, although some mics are quite good too.

Lmao! Best Advice Ever. This is a really cool topic, and something I'm going to be getting into as well. I love music, but I don't understand the technical side of it very much.

Technically speaking, as in method and execution and not the actual theory of composition, here's what I do:
I compose in Anvil Studio, then re-render the midi with high quality sounds using Synthfont, and then master the audio with Audacity, spreading the stereo and bass boosting and all. Note - all free programs. Maybe I will make a post regarding some hints and tips later or something... but oh my gosh it is four in the morning here I really need some sleep

Well what I use to make music is MuseScore because I'm more familiar with notations and whatnot. After MuseScore, I save it as midi and convert to mp3 with Direct MP3 Converter since it has awesome soundfont.

While I've been playing around with musescore I found this soundfont http://www.musescore.org/download/fluid-soundfont.tar.gz
With it and .ogg exporting i never have to export it out of musescore for rendering.

Also I still dont get how chord progression works with solo parts. I see how it works with parts that can play multiple notes at once(I think) but how does it work when the instrument can only play one note at a time?