Expressions can get quite complex and mathematical. If you're not a mathematician , you'll be happy to know you can rely on other people for useful expressions. Since they are just text commands, it's easy to copy other people's expressions, and paste them into your comps . Great sources of expressions can be found at the following sites:
Useful expressions are frequently posted by Dan Ebberts (and others) on the creativecow After Effects forum.
See Dan Ebbert's expression tutorials here:
In addition to these web resources, Total Training's excellent DVD training series at http://www.totaltraining.com ships with an e-book of useful expressions.