First, grab on to the end of the crank and see if you can move it in or out. Then, check the alignment of the ring under no load (put the bike on a stand and pedal - check to see if the ring wobbles). If the arm feels loose or the ring only wobbles under load, you need to tighten the crank arm. If the arm is tight and the ring wobbles under no load, you can carefully align the ring with a large adjustable wrench. Just work slowly and carefully.