welcome to the weight loss forum!
There's some great advice in the replys you've already received, particularly the one about trying swimming.
It's very easy and understandable to think we can't exercise at all when we have an injury or suffer pain in some way, which I know from personal experience.
The trick is to find something you can do.
If you can't walk, cycle (which by the way is good for building the muscles around the knees). If you can't cycle swim. If you can't swim, do weights, or pilates, or yoga.
Also try to improve the situation with your injury, such as trying knee braces, getting physio or massage.
Losing weight doing other exercise (like swimming or cycling) may also help take pressure off your knees so that in time you can incorporate walking and maybe even jogging into your routine.
The most important thing is to not let anything stand in your way of making at least some progress.
Keep looking for ways around problems and you will find solutions.
Good luck with your weight loss and we all look forward to tracking your successes along the way.