Here are a few exercises that will help you strengthen your core
Bicycle crunches
Lie on your back and bend your knees. Alternatively, twist your body to make your right knee touch your left elbow and vice versa
Mountain climbers
Come into a high plank position. Bend and bring your right knee towards your chest, in between your arms. Then bring your right knee back while moving your left knee forward
Double leg raises
Lie on your back with your arms on your side. Slowly raise both your legs off the floor to a 90-degree angle. Lower your legs back to the floor and repeat
V-Sits
Lie on your back and lift your legs and torso, forming a V position. Move your arms forward towards your shin and maintain a good posture. Return to the starting position and repeat