I think one of the best things is to set small goals for yourself first. A big number can seem unachievable, and you might get discouraged and fall into bad habits again. Maybe start off with say, losing 10 lbs in a month? Once you get going, the pounds come off quickly at the beginning because you're shedding water weight, but as you progress, it becomes harder to lose weight, especially if you diet too hard (your body will think you're starving yourself, so make sure to eat healthy and a decent amount of calories).
After you get in to the swing of things, and achieve your 'short term' goals, then maybe you can set a long term goal as well? That way, you'll be able to set something that you know
you can achieve because you've already seen results!
Good luck! And I feel your pain. The doctor's scales are always a bit of a shock...