i think everyone has given you some great ideas- i also second swimming- its GREAT exercise. the water provides natural resistance and engages all your muscles.
i don't have issues with joints, but i started with the mentality of "i can't do this" because "i'm too heavy still". but i just tried my best. i started with 30 day shred (high impact don't do it!) then i got back into my yoga, and i got better at it, and yeah, i was awkward, it hurt, but it got better, i built strength and stamina as time went on. and so will you.
as everyone mentioned, just move a little more. go for a 30 min walk 3-4 times a week to start. see how it goes. then bump it up to a 45 min walk. then try some low impact stuff, like an exercise bike, rowing (like someone mentioned) or swimming.
talk yourself out of the mental block
good luck!!