1. Find the right playbook for your style.
2. Start mastering plays from that play book.
3. Learn your RS audibles.
The only time I usually audible is when i know a play is not going to work. For instance, if I'm running screen and you are in man, you are going to bust that play up, so I'll audible to a run play from a different formation at times.
As for passing, fly routes work really well against zone defenses, and even man if you're not over matched by the secondary. Flat routes to speedy RBs work against most linebackers in man. Button hooks are good vs. zone. Slants work against man, but be careful about the jam or zone. Make sure you see your receiver get off his man before you throw.
The best thing you can do is try to keep your opponent switching from man to zone, run cover to pass cover. Manage every part of the game and try to set things up. And don't be afraid to punt. You don't need to give up any easy points, cause then you will feel pressured to pass every down and it will be over. Good luck.