string conversion and manipulation

Question

I am completely new to C# and fairly new to programming, so my apologies in advance if I can't make sense out of my own question! Here is what I have to do. I need to read a string of 3 characters and convert them according to the following scheme. a=1, b=2, c=3, .....and so on. I also will need to read in a zip code. I don't know if I can or should read the zip code in as an integer. Having read both of these in I then need to combine them into 1 number. For example, I read in abc and the zip code 45678. What I want to end up with is the number 12,345,678. I appreciate any help you can give.