Welcome to cycling. It's a great sport that you can do well into your latter years because it's not bone jarring.
There are several important tips when you're just starting out. One is to peddle in a circle. That means not only pushing down, but drawing your foot back and up. If you have access to a stationary bike, you can try it one leg at a time.
Be sure that your motion begins at the hip. Envision snapping a dishtowel - if you grab it at the end, you get more motion than if you grab it in the middle. When you're riding, envision that your hip is the focal point of the motion.
Keep your feet flat. We tend to ride with toes pointed down. You may have to adjust your seat if it's too high. You'll get much more power with flat feet.
Most important - do not push big gears when you're just beginning. Spin spin spin! Let your tendons & ligaments get used to the motion. Your connective tissues do not gain strength as quickly as your muscles. Even when I'm beginning my serious riding in the spring, I always begin my first rides with just spinning. If you're faced with big hills or headwinds - spin in smaller gears. You won't be fast at first, but you'll have lots less knee pain down the road!
Good luck!