The opening part is played in the first 8 to 10 moves

Begin with a pawn in the middle

Then get your Knights and Bishops  developed  in useful squares

remember to castle to take your King to a safe corner

Beginners work out the  idea behind each move you are playing

Middlegame

Get your rooks Connected
. One of the strengths of castling is that it gets your rooks connected.
Seek an Open File for the rooks
Watch your weak squares.
A weak square is one that can’t be easily defended from an attack. …
Bishops have a slight edge over the knights so if you swap ensure you have a good  reason
Pawn Structure is the key so create and Maintain one