If given the choice of those two, I would suggest you go with the exercise bike. The primary reason for this would be because it is lower impact than what the treadmill would be.
You will most likely be able to go for a longer period of time on the exercise bike without some of the knee problems that people who generally begin aerobic exercise on a treadmill sometimes have.
If you do go with the exercise bike, I would also recommend you "spin" at a faster speed rather than loading up the resistance and "mashing" the pedals. This will give you a nice base for increasing your pedal resistance down the road.
Weights will also be beneficial, however there are a variety of exercises that you would benefit from that require just bodyweight.