Before hiring a plumber, determine exactly what you need: that's going to be an important factor when it comes to deciding which plumber is best for you.
Once you're satisfied that a plumber is qualified and operating above board, you can call around to get some quotes. Be sure to describe the job completely, and be aware that the plumber may need to conduct a home visit to go over the situation and determine if there are any special factors that might affect the price. When you talk to the plumber, ask if she is bonded and insured in case a problem with the job develops, and ask her about her plan for addressing the given plumbing issue you are experiencing.
A good plumber will provide sound, reasonable advice, even if it means not making as much on an individual job.