I am trying to write a Hex(string) to Dec(integer function). The thing is I dont know how to access individual characters from a string. E.G if the string was "9F"

I want to be able to access the "9" then the "f" e.g

firstletter= mystring(0)
secondletter = mystring(1)

but obviously this does`nt work

Help!! Any ideas anyone??