Exercises for someone with a bad knee?

so I really wanna work out more and lose some weight, hard part is.. i hurt my knee like a 2-3 years ago and had surgery for it(tore my meniscus) and i was okay for awhile, but then early last june i went to run and it popped and buckled underneath me..and then it happened again in october to a football game. plus, this time when i go to extend my leg/knee it cracks, you can literally hear it and feel it its sooo gross. so im gonna need surgery again sometime soon to fix that. anyways, i don't know what kinds of activities i can do to workout with my bad knee because it hurts when i bend it,pivot/turn. thanks to anybody who answers ( :