A little help with Converting Hex

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), like in translating games, or something to that purpose. I'm not sure if all Hex can convert to 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