Best approach/advice to hitting?

See ball.  Hit ball.

At the HS level most pitchers have plenty of flaws so I think this comment is sound for your average HS batter against your average HS pitcher.  If you are facing a top prospect in the high 80's low 90's with a major league change and a cutter they are still valid but unless you are in the same category as a hitter success will be pretty tough. 

1. Learn the strike zone and hit only strikes.
2. Be confident of your self and know you can hit ahead, or behind in the count, this allows you to develop patience while allowing you to be aggressive on mistakes
3. Watch the opposing pitchers and your hitters carefully and develop a hitting strategy instead of worrying how many seed you can get in your mouth or checking out the cheerleaders in the front row (What does the pitcher throw, what it the umpire giving them, are there apparent patterns  like fast balls early and breaking balls when far ahead.  Try to understand how the pitcher is trying to get you out so you can counter that. Does he command pitches and look like he has a strategy or is he trying to blow everyone away, can he throw his breaking ball for strikes etc.
4. Have a short memory.  What you did the last time up, the fact the pitcher laughed when you took a swing at a breaking ball in the opposite batters box needs to all be forgotten.  The only thing that matters is the next pitch. 
5. Be ready to hit mistakes hard.  A good "Pitchers pitch" is very hard for even the best hitters to hit with authority.  You get three strikes use them to your advantage. To expand on that idea, good pitchers will take what you give them.  If you want to consistently swing at my fastball at your shoulders you will see a lot of those and if you expand the plate I will be glad to never through you a pitch that even sniffs the white part of the plate. When you force me (the pitcher) to throw strikes in order to get you to swing we are now in a chess match. If you work ahead in the count I may throw for a corner or a knee high and miss with a belt high middle of the plate offering. Be ready to hit it hard.  If you look at tape of hitters in high school and I am nerdy enough to have looked at a lot you will see that in most at bats the pitcher, even good pitchers will provide one pitch the batter can tee off on the key is to recognize it and make the best of it.
6. Finally know the odds and make adjustments when they are in your favor and when they are not.  If the pitcher has placed you in an 0-2 hole, shorten the swing, look to drive the ball the other way, choke up if you do that and look to extend the AB and get back in the game...1-2...2-2...now the odds are back to a reasonable spot.

Best advice, practice your butt off, get good coaching, know your strength and maximize it, know your weakness and minimize it and then (Cliche of the month) Go have fun!!! Baseball is still a game and if you are still playing in HS you have left 50% of the others behind.  Learn to love the game and when you do the rest comes.

Run like hell