I've also got a LOT of weight to lose and I've already lost 10 lbs in about 3 weeks. I started eating less and healthy and I eat fruits a vegetables for snacks or I get those Special K snack bars. You should just eat smaller portions and healthy food and do some walking for now. I wouldn't know how to help you with the exercise thing since you have back problems, you should ask your doctor about that.
You can join this "race" we're having for motivation.
We've yet to set a deadline or have a grand prize for that matter but its already got me motivated and Im pumped to lose more weight. Its easier to stay motivated when you got a bunch of people to "race" to the finish.