You need to implement both a parser and an interpreter.
There is a great free text book called "Programming Languages: Application and Interpretation" that uses scheme to build increasingly more complex interpreters. It also serves as a great introduction to programming language features.
Check it out here: http://www.cs.brown.edu/~sk/Publications/Books/ProgLangs/
If Scheme isn't your cup of tea it may be worth looking into.
