There aren't more than a handful of the players in the Mid-Suburban League who wouldn't see their results improve significantly simply by concentrating on these three principles.
1. Take almost all your time for the game without getting into
unnecessary time trouble; i.e. avoid playing much too fast or too slow. For the overwhelming majority of high school players, playing too fast is a much bigger problem than playing too slow. You are much better off getting into time trouble occasionally than finishing every game with a half your time left on the clock. The bad moves induced by time trouble will be more than offset by the good moves you will find in by playing more slowly.
2. Keep your pieces safe and, win your opponent’s unsafe pieces. Check every move to see whether your pieces and your opponent's pieces are protected. Check every move for basic tactics like forks, pins, skewers, removing the guard, and discovered attacks. Never try to see three moves ahead if you haven't carefully checked one move ahead.
3. Attempt to involve all your pieces at all times.
As usual, my best advice is lifted from NM Dan Heisman's Novice Nook column at ChessCafe.com.