Look under the bike. The lowest part of the engine, sitting between the lower frame rails, is the oil pan. On the front left corner is a 17mm bolt. That's the drain plug. You'll also need to remove the oil filter housing and replace the filter. Look in your manual to see a diagram of the parts you should have in the housing. Many times previous owners lose the thin washer and spring which are necessary for the filtration of the oil.
If you don't have a manual yet, you can download one for free from the sticky thread at the top of the Tech forum.
