Have someone pressure wash it clean, rent a unit if you have to do the job yourself. (If you do it yourself, try to do it at night so the EPA doesn't catch you...). Take any barnacles off with a house paint scraper.
Sand with 80 grit to get a reasonably fair bottom of well attached paint.
Then finish final preparation according to the instructions of the bottom paint you intend to use. Figure out what paint is on the boat and ensure the new stuff is compatible. If you need specific help call Interlux help line and play the dummy and they will answer every question that you may have.
yachtpaint.com - the website of International and Interlux paints