Girl here. I'm not a fan of online dating whatsoever it's arbitrary in my opinion and can make you feel very competitive about love. You have a better chance of getting a girlfriend by trying to gain confidence and talk to girls in real life. But with that being said I do have quite a bit of experience with apps from my younger years. I can't speak for all women but here's a list of reasons why I use to swipe left, even if they were my type:
-Having no bio, or weird things are written in their bio that either isn't funny, too childish, aggressive/victimy or too s*xual
-1st picture or all are group photos, too many photos in sunglasses, subsequent pics are either blurry or trying to hide their face, which is hidden or far away in most pics
-Silly photos e.g wearing Borat underwear, swearing, drunk
-Every pic is a very up-close selfie
-Has less than 3 photos
-Topless photos
-Filtered photos
-Photos in bed (personal preference)
-Gym photos (personal pref)
-Poor dress sense (personal pref)
That's all I can think of for now. Again, don't take Tinder too seriously it's not the real world and only a small portion of women actually use that app. Lastly, you're only 19 you have plenty of time to develop yourself so it's not the end of the world if you're getting rejections now. Hope this helps