Just read you have bad knees. Please don't start to do ANY kind of step classes right now!!!:
Without the right technique and strength you might ruin them!
I am a very dedicated stepper but also a semi-professional instructor. If you really want to start the Step, with your current knee problems:
1. Go to an orthopedic. He can say for sure whether you could do step or not
2. If he says you can, go to a gym rather than do it at home (I realize that if you're very overweight, you might feel uncomfortable in a gym, but have the strength to say: who cares I'm doing this for me!!!! It really helps). They can teach you the right technique (it's not that easy!) to spare your knees.
3. Try to strengthen your leg muscles with special exercises or fitness at the gym. They will help you make your legs stronger and keep your knees in place.
4. Do not put any extra height on your step. It is difficult enough as it is getting thehang of it
I used to have problems with my knees and back because of loose joints and my weight, but due to see above I can do step and enything else I want now.
Don't play with your health!
Be well and keep to your goals,
Good Luck, Greekgirl from Athens