I'm not that high rated but I can give you advice nonetheless.
There are several areas that you can work on in order to improve your "knowledge". After that it's really just about experience and talent.
1. Openings. You will need to do more than know the "right" lines. If somene deviates from the "correct" line you need to know how to respond. The only way I've found to do this is experimentation and (preferably) reading books specific to your opening.
It's important to have at least minor knowledge in most openings, because there is no guarantee that your opponent will play e5 as black or e4 as white. You absolutely will need an opening to play when your opponent plays d4. Some of the more tricky black openings should be looked at to. Such as the French, Caro Kann, Alekhine's, and so forth.
2. Mate patterns. Some come up more often than others, but all are useful. You absolutely MUST learn how to win in king + piece(s) vs king situations. K+Q v K, K+R v K, and K+2B v K are easy to learn in particular. There are tons of books on end game mates, and many books on mid game mates (which are at the least good exercise)
3. End game. This is by far my weak point after being away from the game for a long time. Things are markedly different in the end game, so much so that sometimes your middle game will need to rely on end game knowledge. A lot of this will come from experience playing, but a lot can be learned from books as well.
4. Position. It's important to be able to recognize when your position is good or bad, and what you can do about it. Stranded pawns, dead bishops, etc. can all play key roles in this game. The only useful ways to learn this are experience and books.
5. Analyze your games after they are over. This is important so you not only can look back objectively at certain decisions you made, but also to recognize errors that you or your opponent made, and what you can do about them next time.
In general, books and experience are the only useful ways to learn. I do not personally think that computers are a very good tool beyond answering simple questions about how to respond or play certain lines.
Do you play online? If not, I'd recommend it highly. There are a lot of attitude problems, but it's very good experience. Also, the serious servers will often have lecture bots that are somewhat informative.