Hex (A little help here)

Ok, well, I have converting Binary to Hex to Decimal and back again, but what i would like to know is converting Hex to Letters (A-Z). I'm not sure if all Hex uses is numbers or not, so this might be a worthless post, but I'm trying to learn ASM (Which is very complex, but very interesting for me), so I figure I should take it one step at a time.

Any help? Also, any resources you could point me to for ASM would be nicely appreciated, thanks