You'll need sheets of cardboard, newspaper, lots of leaves and other organic materials, all layered and left to blend together into rich loam. You don't have to be a weightlifter to have a beautiful, healthy garden. But you do have to recognize your body's capabilities and work with them. Prioritize your jobs. Loosen stiff joints and muscles with gentle warm-up exercises.
Backs are a weak spot.
Never bend from the waist to lift things. Bend from the hips or, better still, squat down and let your legs do the lifting. If you're reaching to one side or behind you, turn your entire body, don't twist just one part of it. Keep your elbows tucked in close to your sides when you're raking or moving heavy loads. That brings your upper arm muscles into action, as well as those in your hands and forearms.