It depends on the bike. This type of conversion is always more difficult than first envisioned. Without looking at your bike, here are some potential issues that I'd want to check out.

1. OLD (over locknut dimension). Measure the distance between the stays where your rear wheel fits. You'll need to buy a hub or wheel that matches that dimension or you'll need to mess with your bike's frame.

2. Brake. If your bike has rim brakes, you're golden. If your bike has a coaster brake, you can get an internally geared hub that has one.

3. Cable routing. If your bike wasn't designed for gears, it probably doesn't have any cable guides or stops. Finding clamp-on fittings to match your bike will probably be the most frustrating part of the whole job. I'd make sure that I had the cable routing figured out, including acquiring the parts, before I spent money on any of the other hardware.

4. Shifter. Actually pretty easy. Just be sure to get one to match your hub. Internal geared hubs don't generally respond well to "make do" shifters.