
You are on your back with your legs locked around their waist. Break their posture, then attack or sweep.
Closed guard is the first guard most people learn, and for good reason. With your ankles crossed behind their back they cannot pass, and every time they lean forward you have a chance to attack. It is the home of the armbar, the triangle, the kimura and the hip bump sweep.
The key is posture. If they sit up straight, your attacks run out of reach, so pull them down with your knees and grips first. Keep your hips moving and chain your attacks: the defence to one move usually opens the next.