To play on a square, a player needs one of their own discs on the far side of the discs to be flanked. This disc is called a foothold. If you turn the foothold into your disc, the opponent cannot play on that square. If several moves defend, choose the quiet one among them.
Usually you look for taking first, and use defense when taking first is impossible or would spoil your shape. But taking first is not always the first choice; as you improve, you will sometimes choose defense first depending on the position.
Exercise
White to move. Black would like to play B4, but White cannot play B4. Of the marked F4 and C2, which move stops Black from playing B4?
Exercise
White to move. The square the opponent most wants is one you cannot play. Of the marked E1 and G5, which move stops the opponent from playing there?
Exercise
Black to move. The square the opponent most wants is one you cannot play. Of the marked A5 and F3, which move stops the opponent from playing there?
Exercise
White to move. The square the opponent most wants is one you cannot play. Of the marked D8 and E3, which move stops the opponent from playing there?
Exercise
Black to move. The square the opponent most wants is one you cannot play. Of the marked F7 and G3, which move stops the opponent from playing there?