Plank is the best exercise for strengthening your core. Remember that the core is like a corset and it pulls in from the front and the back of your body. A strong core will aid in all other exercises. It will slim your waistline and make many day to day activities easier.
Place your elbows under your shoulders. Start on your knees and bring hips off the floor to form a straight line from shoulders to knees. If this is too easy pop up onto toes and hold.
There should be a straight line from your shoulders to your feet.