The best place for starting to learn how to write expressions is Dan Ebberts' Motionscripts Mastering Expressions.
http://www.motionscript.com/mastering-expressions/table-of-contents.html
Dan also is a frequent poster here and can answer almost any expression related question you have.
I also like School of Motion 30 Days of After Effects on YouTube, which have quite a few lessons that walk you through every step of using expressions to achieve a certain effect. Workbench is also a good resource on YouTube.
As to your expression question, I'm new to expressions as well, so I'm looking forward to somebody, probably Dan, answering that question.