The chess middlegame becomes much easier if you have some ideas to work with.
The middlegame starts where the opening finishes. As soon as one of the players has finished the development of his pieces and made his king safe. This player may then start attacking something in the enemy camp.
Trade your bishops for knights in closed positions.
Trade a knight for a bishop, winning the pair of bishops.
Trade your queen for two rooks.
Trade your rooks for a queen and one (or more) pawns.
Occupy open files with your rooks
Give your opponent an unfavourable pawn structure.
Make a positional sacrifice
Make an exchange sacrifice
Exert pressure on a certain target
Gain the initiative
If your opponent has the two Bishops, trade one.