When I started exercising regularly I was around 290. I started with 3x a week of activity. Usually it was either water aerobics (we had someone in the class that couldn't swim and just stayed in the shallow end) or walking on the treadmill - I would do intervals like c25k but instead of running I would just crank up the incline and/or speed to a faster walk. Sometimes I would also do zumba, spin, or the elliptical. When I started running I used a plan on sparkpeople's site that was a little more moderate than c25k. You just have to work around what you can do, what your limitations are.
I didn't start using it til later but I think the rowing machine is a great full body workout that is easy on the joints. Even just throwing in 5 or 10 minutes with whatever else you are doing is good. Weather permitting it's nice to get outside and walk, hike, whatever as well.
Also strength training, if you have a gym membership you can see if they offer giving you a tour around the equipment and making some suggestions, otherwise easy enough to search the forums for starting ideas and search youtube videos for proper form.
And obviously as you mentioned, consistency. Since late-Feb or so the only time I didn't go was from 2 oral surgeries, on the surgeon's orders to take a week off each time. Otherwise I'm there regularly and just gradually built up. I've had the occasional injury and had to change what I'm doing, but kept doing something, first the 3x a week, these days usually 6x a week so I have 1 rest day. Now I can get on a treadmill and run for a few miles (granted I am not the fastest runner

) before I need to take a 60 sec walking break. It just takes time and patience.
I'm not sure if you mean exercise isn't doing anything as far as weight loss goes, or you just aren't feeling any improvements. If it's weight loss though, chances are it's something with the diet that needs to be tweaked!