There is a saying among bike mechanics that "the most damaging person in the world is a novice with a spoke wrench".
Well, we were all novices at one time and this isn't that difficult to do properly. I second the recommendations to get a good book on wheel building and/or read the Park section on wheel building and truing and Sheldon Brown's article on wheel building. It is important that you have a clear mental image as to how the spokes are patterned and how they interact. Truing is a systematic job, not random tightening of spokes.
Other suggestions:
Go slow. Turn the nipple(s) you are working on only 1/4 turn at a time. A small adjustment can make a significant change in the rim position. You are not winding a clock.
Start at a known location so you can find the same spoke later. Mark the rim with a pen so you know where you were.