Cardio suggestions

It depends on what the problem is with your knees. Can you ask your doctor for a physical therapy referral, and work with the PT people to find exercises that are doable for you?

I have a torn meniscus in one knee, and some osteoarthritis in both. I find that pain or strain is way too frequent if I do things involving much impact, or torque on the joint. That means that things like running, jumping, things like Zumba or martial arts, etc. are not a good plan. However, what I can handle is exercise that involves straight-line hinging motions at the knee, without impact. That means biking, spinning and rowing are some things I can do.

If your primary goal is weight loss, then managing the eating side of things will make the biggest contribution, and the weight loss itself may reduce your knee pain (it reduced mine).

Cardiovascular exercise is good for health, and gives you a few more calories to eat. Any exercise burns calories, include seated ones that use only the upper body: Chair exercises, seated upper-body weight training, arm bike, etc. Some people find swimming or other water-based exercise to be compatible with their knee problems. But what works, or causes trouble, is pretty individual.

Best wishes!

What AnnPT77 said. Get a recommendation from your doctor and/or a physical therapist (who would probably be the better option I suspect but I wouldn't be shocked if you'd need a referral to see a PT). Swimming can be great, until it isn't. The same is true for any other activity. There are strokes that I can't swim more than 100 yards of at a time because of how badly the affect my knees (well ok *a* stroke).

In my case weight loss wouldn't have helped my knee issues but that's because they were related to how the patellas moved, not how much weight was being put through them.