Use a glass, not a plastic, bottle. Some plastic breaks down into toxic substances, but glass is made of silicon (sand) and won't hurt the environment. Use a rubber stopper or cork; metal caps corrode in saltwater. If you can, seal the stopper with wax once the note is inside.
Next, find a current. You need to look at a chart. A ships' chandler, marina, or marine-supply store can help you. Most world atlases also show major ocean currents. (Information is also available from Woods Hole Oceanographic Institution in Woods Hole, MA, 02543.)
Though a river current can carry a bottle out to sea, a bottle dropped into the ocean from a boat has the best chance. The Gulf Stream runs very near the beach off parts of Florida, though, so a bottle dropped there may go all the way around the North Atlantic Subtropical Gyre.
The second-best choice is to throw the bottle into an ocean inlet during an outgoing tide. (Check tide and current tables.) Cape Cod, Cape Hatteras, the Bahamas, and elsewhere have ocean inlets. The tide pulls the bottle out, and keeps it from being washed back to shore by the waves. Be sure the wind is blowing away from the shore.
Include your name, address, location, date, and time you dropped the bottle in your note. Use a permanent marker or ballpoint pen.