I think you're being really smart about wanting to exercise now so you'll reveal a healthier body as you lose weight. And you're right about the scale; it's only a tool, and if it's one you don't feel is helpful to you, you aren't constrained to using one. (As an aside, I used to feel that way about scales too, but I'm now glad I got one because daily weigh-ins have actually put me more in tune with what's going on in my body--but they are absolutely NOT mandatory.
It's impossible to say how many pounds goes into a particular size. I weigh 208 pounds, but I can shop in average-size sections of the department store. Other people my height and weight are shopping in the petites section or the plus section, and it's all because of how and where we carry our weight.
Scales are just a tool that provides you with a number, but clothing sizes are...well, they're just a different kind of number. It's better to look great in an 8 than bust your *** for a 4 when vanity sizing has made sizes meaningless anyway (when I weighed 125 pounds many years ago and vanity sizing was just ramping up, I fit into everything from a 6 to a 12, so it's not like sizes are uniform enough to use as a guideline).
As for exercise, you'll naturally look tighter just from losing some of the weight that's hiding the musculature you already have. I've had great results from working with free weights, but others could give you a lot more suggestions here. I'm a real novice at exercise of any sort.
One thing I have picked up, though, is that we shouldn't be afraid to lift heavy because we're female; it takes serious
work to build bulky muscles, not the three or four times a week that I do. I am in no danger of developing Schwarzenegger arms from my fifteen-pound weights and some triceps dips, believe me.
Good luck, and I look forward to reading of your future successes!