...figuring out chord progressions to some songs you like.  Many pop songs have pretty simple progressions - the most common of course is I-V-I or I-IV-V-I or I-vi-IV-V-I (every hear or play that tune "heart and soul"? there it is).  All these roman numerals refer to are the scale degrees on which a chord is built.  So if you're in the key of C and assign a numeral to every note in the C scale, I would be C, ii would be D, iii would be E, etc.  So if we wanted to do a I-V-I chord progression, the first chord, since it is I, would be a C major chord, the V would be a G major chord, and the I is C again.  The definition of a chord is three or more notes sounding at once.  Most chords start with what is called a triad (or three notes).  Visually, if you are looking at a staff, this translates to stacking notes on top of each other, like a line on line on a line.  So a C major chord is C-E-G (every other note in the scale).  A G major chord would be G-B-D.  The reason some roman numerals are upper case and some lower case is that upper case numerals denote it's a Major chord (happy) and lower case ones denote a minor chord (sad).