In the US, I believe eviction law (etc.) is state-by-state, so it's worth checking what the rules are in your state. Many years ago I evicted a roommate (in NC), and I believe it took 3 months, start to finish, and required that he violate a written contract (which he had, by not paying his rent to me on time). It was a pretty unpleasant 3 months (and yes, better checking references, etc. probably would have spared me -- hindsight...). Procedures others have noted (e.g. month-to-month/30-days notice) may also work, but it's worth checking.
Lawsuits: you can talk to your insurance agent about what sorts of protection your homeowners/renters insurance would provide. If a pipe bursts and ruins your roommates heirloom quilt, are you liable, and for how much? What about if they slip on the snow because you fail to shovel the walk? I wouldn't sweat this too much, but it's worth knowing what sorts of things you might need to protect against. Bear in mind that if you don't have much in assets/insurance, suing you may not be all that attractive, as long as you don't do something truly negligent (and maybe even if you do). It may be useful to write things into the rental agreement (e.g. to note that renter's belonging's aren't covered by your policy, if they're not), just so you have a record.