It's worth paying for a good tune up the first time, just for safety's sake. After that, find a set of basic tools on sale (I paid $35 at Nashbar) and pick up a copy of Park's Big Blue Book of Bicycle Repair (my personal favorite) and learn to do it yourself. It's simple, satisfying and pays for itself quickly. It also saves time since it takes less time to fix stuff on a bike yourself than it does to wait for someone else to do it.