In the beginning of the game one develops the pieces, controls the center and safeguards the king. But after these 5-10 opening moves, which many players play by memory, one really needs to think. Here is how to handle this part of the game.
1) Look out for tactical opportunities. Are there any unprotected pieces of your opponent, that you could easily capture? Any opportunities for forks, pins or skewers? On the flip side, make sure you do not give your opponent any such opportunity. If such an opportunity already exists, make sure you fix it.
2) If there is no tactical opportunity, consider the long picture. Are your pieces in the most active squares? Are rooks in open files and bishop in open diagonals, ideally as long as possible? If you have only one bishop, make sure the pawns do not block the bishop, try to have pawns at squares of the opposite color. This is especially true if both sides have one bishop each, of the same type (light or dark). Try to have your pawns at squares of the opposite side, while forcing your opponent to have pawns at squares of the same color.
3) Pawn structure. Try to have as few pawn islands as possible. Avoid double pawns and backward pawns. Try to have your opponent face such issues. Try to create a passed pawn, ideally supported by another pawn.
4) Eventually try to reach a winning ending, or at least an ending where the odds are on your side.