as spike says asc("A") is 65 - so you can convert a letter to a number by
asc(mystring)-64
Except ... by some magic access is able to sort "A" and "a" as being the same - but the ascii code for little a, is a different number to big A -
so you had better use
asc(ucase(mystring))-64
note that you can turn it back with chr() function
mystring = chr(somenumber+64)
also somewhat irritating having to use strings, when you mean a single letter - as unlike many languages, vba has no concept of a char. (single alphanumeric character)