Assuming you want to keep the brakes and gears in the flat position, you just need bars that are the right fit for your stem. Unattach everything, remove old bars, put new bars on, reattach everything.
If you want to change to road levers with STIs, that's a lot more complex.
http://www.bicycletutor.com has a lot of info.
If you really don't know what you're doing, find a friendly bike shop!