My Nissan altima knocks when started. It also burns oil, I am afraid it might be the piston rings. Tell me if I'm wrong

Unless the vehicle has been neglected, it's hard to believe the rings are shot. Remove the oil cap and look inside at the valves and run your finger along the valve cover. How do things look? If you suspect your engine is sludging up, I'd recommend you read about Auto-RX. Go to BITOG.com and follow the clean and rinse phases. After you clean your engine, start using Valvoline MaxLife Synthetic, which is formulated to keep higher mileage engines clean. Change oil and filter every 5,000 miles, thereafter. Also, check out your PCV valve/hose and EGR valve.