There are a lot of good choices. Go to a music store with a price range in mind. If you need to keep it under $150, the Shure SM-58 is an established favorite, for stage especially, but also for recording. At higher prices, you can get a condenser microphone that requires a power source but are generally thought to be the higher class of vocal microphones.
One thing worth adding... a good singer will get through, regardless of microphone. Of course it's always good to have good gear too.