All said here is good advice, and hopefully read by our young players.
I'd like to focus on the pitcher assesment part. because I think this dictates most hitting and approach. Assesing the pitcher is relative to the batters skill and ultimately his confidence.
I doubt Ted Williams, Pete rose or Barry Bonds assessed pitchers the same way but they did something all good hitters do. They went up their with a good idea of how they matched up against a pitcher relative to their hitting style. Subsequently, their approach was more or less aggressive.
Ofcourse good team baseball requires situational awareness but the permutations of all that are too many to write in a thread.
That said, I'd like to concentrate purley on the beating the pitcher aspect - either getting a hit or walk or sac.
It's all about preparation which leads to confidence. I tell kids look at the pitcher and first ask yourself
1. is he likely to strike me out?
2. Does he have command?
If you know your relative ability and the answer to these two questions you pretty much have your hitting approach narrowed down about 75%.
1. If he is likely to strike you out and has command then be aggrssive looking to swing early and often. otherwise he's ahead in the count and Oops!
2. If he has the stuff to strike you out but has questionable command (these are often the toughest)- make an early decision on high/low and be controlled looking to get ahead in the count - then be aggressive. Bad swings are made at high or low pitches. In or out and knee to belly B can be hit.
3. If he's unlikely to strike you out - look for your perfect pitch on strike one and two and relax knowing you can put strike three in play.