Alright, you're sold - dancing is for you. But...
Standard disclaimers. I'm not a physician, and if I were I don't know jack about you anyway, so you know the rules - see a doctor before starting any exercise program.
That said, it can be as much or as little exercise as you want. If you're recovering from years of couch-potatoing, the easiest place to start would be with smooth dancing like foxtrot. It's slow, and in its easiest forms is much like walking. Rumba is slow too, but there is more lateral motion which might bug your hips. Bad knees might slow you down a little, but then you might strengthen them too if you give it a chance (yeah, I'm talking to you...).
If you're in good shape already, the place to start is probably with east coast swing, or what the locals call Imperial swing after a long defunct ballroom. It and variants like jitterbug can go about as fast as you might want to. Cha-cha will also get you moving, and it'll loosen up your hips if you do it right.
Alright, you know what to learn - now where to go? If you're totally green, a local community college is probably the best place. That way classes start everybody in the same place. You might think that would go without saying, but that's not necessarily true at a dance studio. I know I've walked into some nominal "beginner" classes that IMO were too advanced or else moved too quickly for raw beginners. It can be very frustrating - I for one am not used to being at the bottom of the class, especially in one where everyone else can grade you.
Or if you'd rather limit the number of people who witness your development, there's always private lessons. Maybe you're lucky enough to have a friend who knows a few things so you can get by cheap. In the worst case, here in St. Louis private lessons from certified teachers have run as low as <$30 per hour with an extended agreement and a new studio, but you'd better budget at $50.
But you don't have a partner? No problem. There are usually extra partners of one sex or the other in a group class, usually women, and instructors usually have you rotate partners so nobody stays alone for long. You might wind up dancing by yourself part of the time, but that lets you watch the others and practice by yourself until you get a better feel for what they're teaching (or just convince yourself that the others are having trouble too). Also, instructors might try to shuffle people around between classes to even things up, and usually the women are willing to dance with other women if one of them knows the man's part of the dance.
But the other dancers are older/younger/uglier than I am! So what? - I'm assuming that you're there to learn to dance. Assuming that the person isn't so fat or feeble that they literally can't move, you can't tell much about who the good dancers are just by looking at them away from the dance floor. And they won't all be particularly attractive. (Yo young guys in particular - remember the wise counsel of Benjamin Franklin (beware of a tame nude picture). Things you learn with the older women just might come in handy with other women). But I've found dancers to be a friendly accepting bunch, and assuming you possess rudimentary knowledge of manners, dress and hygiene, and you put forth decent effort to learn, you'll do fine.
Now it's time to hit the street. Dance studios and other organizations often sponsor dances to drum up business. My experience is that as long as you don't pretend to be something you aren't, your partners will be very understanding with beginners. Then you have an instant excuse to meet anyone you want for three minutes of intimacy.
Alright, you've run out of excuses. Out on the floor, now!