This shouldn't be very annoying - just slightly annoying. RedDane had it right - the MRI room is often cold. You'll want to wear a long sleeve t-shirt, some sweat pants and some socks. You can't wear any clothes or anything else with metal in it (no glasses, zippers, etc.).
Basically, they'll have you lie down on a table on a track in front of the machine. Wear the ear plugs they give you and make sure they are in tight and that you are in a comfortable position. Once you're in place and ready they are going to put a frame around your head to keep it still. At the start of the actual imaging, they will move the table on the track so that you are moved into the machine. The machine itself is extremely loud and makes lots of weird sounds. Just relax and you'll be fine.
Good luck!