The first kind of cipher you are going to build is called a Caesar cipher. Specifically, you will take each letter in your message, find its position in the alphabet, take the letter located after 3 positions, and replace the original letter with the new letter.
Start by finding the position of the first letter in the string. One way is to use the built-in find() function:
Above, char is the character you want to locate, and a_string is the string you want to parse.
At the end of your code, call find() on your alphabet string and pass text to the function.