Unless the bike was modified, it's highly likely that you have a 1" threaded fork. If that's the case, you can't just buy a threadless stem and put it on (unless there's some sort of work-around that I'm unaware of.) You would need to buy a new threadless fork, and a threadless headset.

Getting a quill stem with a removeable faceplate as suggested is a great idea to be able to swap bars. Just measure the rise and reach of your current stem and buy the new one accordingly. Of course, you can get a stem that has more/less reach/rise depending on how happy you are with the positions offered by your current stem. Also measure the diameter of your stem, or the ID of your steerer tub. The new stem must have the same diameter or it won't fit (or it will be wobbly.)