But, to start, you might want to pick up The Past Through Tomorrow, a collection of short stories and novellas that pretty much defines his Future History. From there, you want to read Starship Troopers, and The Moon Is A Harsh Mistress, and the "Boy's Life" books, of course.
Jen, what do you suggest?
***rummages through the gray matter***
Didn't I read Starship Troopers and decide I didn't like it? Seems like I did a couple of year ago at thebeach.