The study of the middle game is different from that of the opening. In the opening it's useful if you study and understand all the theoretical possibilities. In the middle game, however, you cannot study all the theoretical possibilities due to the great complexity of the game at this point. That is why in this part of the game you have to know how to evaluate the position. If you read piece hierarchy you already know how to evaluate the strength of your pieces. Your situation in the game is not only given by the number of your pieces and their strength but also by their mobility and their position on the chess board.
Once you've estimated your situation you can start developing a plan. But what kind of a plan should you do? What exactly is this plan? Your plan will be composed of a succession of strategic operations. It could be large and very complex or it could be smaller and simpler. When making the plan you can decide whether you should attack or defend. You can also decide whether you will exchange pieces so you can get to the end game faster. Your plan will almost always contain other smaller plans. They might have as their objective capturing an enemy piece, isolating an enemy piece, controlling some strategic square or some other immediate action.
When you make the plan you have to take in consideration the main element that characterize the current position. For example: if you have an extra piece than your plan will have to focus on taking full advantage on that extra piece; if you have some positional superiority you must use it in your advantage. On the other hand if you have a weakness your plan has to focus on strengthening your game and eliminate that particular weakens.
You should always remember an important rule : if you discover that you have some sort of superiority in the game than you have to take full advantage of it immediately and not wait. If you wait and don't attack, your opponent might find some extra resources and reinforce his position. On the other hand if you have a weakness you have to play in such a manner that you will eliminate that weakness.
A good way you can protect yourself against enemy attack is by exchanging the pieces (this is usually a good thing to do only when your enemy has a positional superiority; if he has an extra piece then by exchanging the pieces you will usually make him a favor and take the game faster to the endgame where that extra piece will make a huge difference).
In order to make your plan you have to be familiar with tactic and strategy.
When we talk about strategy we talk about establishing the purpose, about developing the long term plan. Strategy is a more abstract notion than tactic. Tactic involves the short term plan; it's a solid notion that usually has as its final purpose gaining an immediate advantage.
Many new players spend a lot of time learning a lot of strategy notions and openings. My advice to you is to first start learning tactic. This way you will escape from the rigid rules of strategy and learn to think independent of those rules.
Don't get me wrong: strategy is not a bad thing; Because it is an abstract notion it's just a good idea if you start studying it more seriously later on, after you get more comfortable with chess. Remember : in order to have great games you have to combine tactic and strategy; this way you'll know how to make your long term plan and how to immediately take advantage of your current position.