You are wise to consider exercise an important part of your new lifestyle-by exercising regularly you can limit the amount of muscle loss during the weight loss phase.
The best thing to do is have a good plan for consistently changing the part of the body you are working and to have good variety. Keep the body guessing what is coming next....in P90X it is referred to as muscle confusion...and it has worked very well for me.
I started cardio and weight training in August 2008-6 months before my surgery. In the beginning, I would do upper body weights one day, then cardio the next, then lower body weights, then cardio, then yoga, then running, then a rest day. Now, for weight work, I primarily do only upper body and use my running/hiking for my lower body work....my leg muscles were getting too big and it was making my pants legs too tight.
Try to start out with 5 lbs. If that is too heavy, then back down, if too light, increase a bit. The most important thing to remember in weight training is that proper form is crucial-and you should "feel" the last 3 in a set....so if you can do 8 in a set, and feel the last 3, you are good. You'll want to increase your weight when you no longer "feel it", but always remember that you must keep good form.
I'm more for increasing weight rather than trying to do a bunch of sets. I do a max of 3 sets of 8-15 depending on the exercise. I use 15 lb weights for most of the upper body exercises. If I use the Nautilus machine, I generally use 40-60 lbs for those exercises. I prefer free weights over the machines.
Wait until you get clearance from your doctor to begin and remember, the key to a long lasting exercise program is finding things you like to do.
Let me know if I can help in any way.