I lost 100lbs now i have low bfp, and have a lot of muscle mass but have too much loose skin to fill with muscle. how do i tighten my loose skin?

Several things: If the weight loss if very recent, i would give it a few months to see how your skin does on its own because you are young and your skin is elastic. If at the end of that, there is still quite a bit of loose skin, there are skin tightening light devices or even plastic surgery to consider. But give it some time, don't rush into procedures. And congratulations on the great achievement!

Loose skin: At 17 i would be patient you may have more retraction of your skin. If that doesn't happen in a year or so you may consider surgery to remove the excess skin. Unfortunately this will result in a scar usually in a place that is covered by your under garments.