I would try three days a week cardio, two days weights. Start out at about twenty minutes of cardio; if you can do thirty thats great, but don't overdo it. I like to do the eliptical machines; those might be good to start with since they are low impact. If you feel any joint pain or anything you might have to start slower, but cardio three times a week shouldn't hurt you too bad.
Also, as far as motivation goes, I just want to say you've got to force yourself to go! Once you get there religiously for a week or two, it will become a habit and you'll feel funny if you don't go. Just remember that it gets easier- its all uphill once you start!!