Not really on topic, but that doesn't seem to be too important in these forums...
I too have knee troubles, and cycling is problematic. Depending on your situation, you knees might not spell the end of your cycling days. Physio and careful exercise have got me back to the point I can do some (careful) cycling again. If that's not an option for you, you can ignore the rest of this post.
If you are pedalling your bike, you need to be very careful about the size and fit of your bike. It has to have a tall enough frame for the length of your legs, and the seat must be far enough back for the length of your femurs. If either of those are wrong, you'll put extra strain on your knees.
A lot of people go with mountain bikes when they start to slow down, because of the more upright posture, but mountain bikes tend to be a bit under-sized (when you're on a rough trail, you want to be able to easily put down a foot). Today there are lots of alternatives in the "comfort cycle" class that are probably a better choice for casual cyclists.
You should be cycling with the ball of your foot on the pedal. Use loose toe clips if you have trouble keeping your foot in position. With your foot in this position, at the bottom of the pedal stroke you should be able to almost (but not quite) fully straighten your knee with your foot/ankle in a neutral position. You want a bike frame that is big enough to let you do this without raising the seat too high (if you have to raise the seat too much, you might not be able to get the handlebars high enough to give you a comfortable posture).
You also need to make sure that the seat is positioned correctly front-to-back. When the pedals are at the 3- and 9-o'clock position, your leading knee should be directly over (not in front of) your leading toe. It's not obvious that this is important, but it really is -- very hard on the knees if your seat is too far forward.
Once you get the seat in the right position, then you need to get the handle bars positioned for a comfortable posture. If you're a casual cyclist, you don't want to lean forward too much, because with occasional cycling to don't adapt to the weight on your arms and it will give you hand/wrist/elbow pain.
As in most things, the key is to know what you are doing (or go to someone you trust who knows what they are doing) to get a right-sized bike.