Visual Basic Question

HELP!! hahaha, If your reading this thank you. Here is my question, im writing a decode program in VB. Basically I want the following...
(example)
a = 1
b = 2

now, originally i was using cases. the output would give me the following....

a <-- assuming that this is a text box
1 -----> output of the above

b<-- assuming that this is a text box
2 ----->output of the above

But it will only work with single Letters, I want to be able to out put the following

ab <--- input
12 ---- output

My friend said that i need to be able to read character by character so please how do i do that? or if you have any advice or questions let me know, im stressin out hard. thank you everyone ahead of time.

Use the Mid function:

MyInput = "abcdefg"
extraction = Mid(MyInput, 4, 1)

Extraction now equals 'd'

Wait, I dont get it, How am i going to set that up? I dont it it to Equal D, i would want it to equal the Number, if i had ABCD, i would want the output to Equal 1234.