next main goal after the first ten moves is
to control the center. If you control the center, you will gain space
for moving your pieces, make them very powerful, and reduce your opponent
to a defensive, cramped, position. If you launch an attack when your positions
are equal, you have an equal chance of winning. If you launch an attack
when you have an advantage, you have a much better chance of winning.
For this reason, it is better to not attack until you have
control of at least part of the center.
the center" means "to be able to put a piece anywhere in the
center without it being driven off or traded." If you are lucky enough
to control the center, you can do anything you want to do. More often,
though, the best you can hope for is to control one of the center squares.
But if you own one square in the center, you can use it as a stepping-stone
to get to other places on the board. White ususally tries to control
e5, while Black usually tries to control d4.
Never let your opponent own a square in the center. Always
challenge any square he's trying to control. (You challenge a square by
attacking it with a piece, or even better, with a Pawn. That way, if he
tries to put a piece on it, you can exchange or even capture it.)
If you develop your pieces with the center in mind, your job during the middle game will be much easier.