So, i want to use "inString(" command and "sub(" in ASM, and i need someone to show me the routine. I dont know why, but the one on the "Learn ASM in 28 days" isnt working. Is there other way, using the null string?
HL=address of string to look for
DE=address of string to find
A=positon

I think this is around what you want:

;############################
;input: hl = start of string
; de = string to find
;output: hl = start of string in string
; b = offset to start of string
; carry if string not found

inString:
ld b, 0
inStringLp:
ld a, (hl)
or a
scf
ret z
ld a, (de)
cp (hl)
jr nz, notAMatch

push hl
push de
checkMatchLoop:
inc hl
inc de

ld a, (de)
or a
jr nz, notTotalMatch

pop de
pop hl
ret
notTotalMatch:
cp (hl)
jr z, checkMatchLoop

pop de
pop hl


notAMatch:
inc b
inc hl
jr inStringLp

I could make it faster, but it would be bigger, so it's a trade.

Edit:
    After reading that section of 28 days, I should mention that you need to use zero terminated strings for this. You will almost never use length prefixed, so don't worry about that.

It's not hard to do in Mimas, just do this:
.db 5
.db "Hello"


I noticed a while ago that the routine supplied in 28 days doesn't account for partial matches. There are also a few other errors in some routines...