When do you need a lawyer?

You need a lawyer for sure if you get arrested. You're probably going to need one if you get sued. If you think you're going to get arrested or you think you're going to get sued then you probably don't want to wait -- shopping for an attorney is much easier before you get arrested than it is if you're already in jail.

Other than those times, when do you actually need an attorney?

Lawyers.com has some guidelines.