I too recommend a lake or pond for a few hours before setting afloat in moving water.
If you just have to go on a river, remember one simple rule. Point the bow of the boat at a 45 to what you
DO NOT want to hit, and pull back on both oars. This is the easiest way to apply the brakes and avoid most obstacles.
The book on driftboating is highly recommended as a good place to start. Of course nothing replaces hands on learning.
As far as coating the bottom, most rivers are running pretty good right now with good solid flows. Not much to worry about bumping on or sliding over. Any river that is dropping and clearing that fast usually is not a good place for a beginner to try and drift. See the posts on the Necanicum to get an idea. Places like the NF Lewis, Sandy, Clackamas, Wilson have some good beginner type water. Never floated the Washougal or East Fork , so I can't really say how they are to float.
Enjoy your boat, and get out and whack some fish !
Chris