When is it good to have an open/closed position? and which side to attack?

how do I know when to close the center/ open it? and how to know which side to attack? and do we always attack the king?

I am not an expert in this topic but i always follow on how safe my own king is. If my king is safe , then only i look up for attacking my opponent and creating complexity.

and do we always attack the king?
No, attacking the opposite side is also one of the big tactics and can be widely used for destroying the opponent's king safety

@WassimBerbar Generally if your king is in the center don't open it, especially with black. But there are openings where they do this on purpose like the Scotch or Stafford Gambit.

If you are attacking on one particular side (kingside or queenside) a closed center would help you since the opponent will have difficulty bringing his pieces over.

Mostly opening the center after developing is castling is considered good. But again, depends on the style of the game and the opening.

when to close the center/ open it?
Depends on what type of positions and openings you prefer to play . General rule of thumb is

1. When opponent is having a play on flanks , you open the center and when you are playing on flanks you close the center .
2. If there is Bishop and knight imbalance . Side with knight tries to close the center while the one with bishop should try and open it .
3. It gives you a material advantage .
4. To free your pieces .
   And many others .

It's always a judgement call, but generally you want to open the centre if 1) you have the bishop pair, if 2) the opponent's king is (temporarily) stuck in the middle or if 3) you have a lead in development. And the reverse if your opponent has 1 & 3 or if your king is still in the middle.

1 is usually a gradual process to get the maximum benefit of the often only theoretical advantage of the two bishops.
2 & 3 can motivate sacrificing material to achieve it - since those advantages are usually temporary. But they can also be traded for another advantage like creating a weakness or a favourable trade of pieces, etc.