Read some tips on how to get rid of stretch marks on this site

Start your skin treatments now as you are losing the weight. Use pure cocoa butter on stretch marks and areas prone to stretch marks like breasts, underarms, thighs, stomach. Skin treatment must be done every day, and it may seem annoying at first but its essential and will become habit right after your shower or sometime. Use tightening/anti cellulite cream like l’oreal sublime slim and massage into skin daily. You should even do it twice a day. Other good toning skin products contain seaweed, caffeine, viatamin E, Cocoa or coconut oil, collagen
You can also find bath oils or mixes at health stores that contain vitamin E, caffeine, or cocoa oils and I would suggest soaking in them or using soaps with them in it.
Also eat alot of protein to develop muscles and its good for your skin as well

Use firming lotion, it might help. There are so many products available in the market.

How much your skin tightens after such a significant weight loss depends on your age, and how long you were obese. Skin is very elastic, up to a point. But like any rubberband that has been stretched too long, the collagen fibers break down and you lose that elasticity. For a 100 lb weight loss, it is likely that you will need some surgery, especially around the abdomen, to tighten the loose and soon to be saggy skin. There is no exercise you can do, no cream you can apply or pill you can take that will make your skin more elastic.