I was in your situation before man. Throughout my teenage years I was convinced I was very ugly, like a 3 tops. I was extremely shy, couldn't talk to girls, often depressed, convinced anybody who spoke to me or was friendly towards me was doing it as a joke or prank and was ashamed of pretty much everything I did or enjoyed.

That lasted until I was about 19 (21 now), it was about then that I started working towards improving myself. For my physical appearance I bought weights and started working out with help from youtube videos and online guides (currently trying to reach 100 push/sit ups), I bought clothes that fit me properly and accentuated my strengths and my own personality rather than trying to copy what others wore, I started putting effort into my look by testing out different hair styles and looks without caring that others might think me too camp, metro or whatever, and I ensured I kept my hygiene as high as possible.

All this helped my confidence but not as much as changing my attitude did. The main two ideas that I hold onto are that it could always be worse and that you don't have all the time in the world. The first is very easy to observe by comparing yourself to idiots, stupidly obese people, and ugly/deformed people and you'll soon realise you've got quite a cushty life in comparison. It's not nice but it helps. The second one isn't just to be afraid of your own mortality but rather to embrace it. You've only got one shot at life, why waste time crying about it? Get on with enjoying what YOU enjoy and being the best you can be. The second thing to take from it is that nobody cares about the stuff you do, they might notice here and there but really everyone is too self-absorbed to notice your fuck-ups, and even if they did, everybody fucks up from time to time, nothing to be afraid of.

Lastly you need to get a hobby that you're good at or can become good at, essentially something you can work on and improve at. For me one of my things was League of Legends, I started in the bottom 98% of the player base 3 years ago, but when I stopped playing I was in the top 3%~ of the world's entire ranked playerbase. Now even though it's just a game, means absolutely nothing and isn't seen as much of an achievement to most folk, to me it was something I could work on, improve, and become the best in my circles at. Which I did, and it emphasised my other efforts to improve myself as not only did I feel like I did well but friends and others also commented on how well I did, and it wasn't a terribly hard thing to achieve.

From there I was confident in myself and my abilities but still wasn't the best at talking to girls. How I fixed that problem was by simply making friends with girls who I had absolutely no attraction to or who were out of bounds. It's easier to do this in a workplace (particularly retail) rather than school because you're on a small team who are supposed to work together rather than a random social hive but it's still feasible considering group projects, after school groups and such.

To build on from this I also started speaking and making friends with anybody I could, not because I was particularly interested in being their friend but rather because being socially active with people regardless of gender is not only mentally healthy but is good practice for people you actually want in your circles, and by having larger circles with more friends and acquaintances you then gain more opportunities to expand those circles even farther.

I hope this gives you some ideas about how to solve your situation or helps you with some thoughts.