I'm almost certain I have social anxiety; I do crazy things to avoid my fears, like wearing a jacket all through summer to try to hide that I have skinny arms, I always say the wrong thing in social situations, I analyse everything that I have said in a conversation later on and cannot think about anything else, and the thought of going out with friends terrifies me.
But I'm not sure I'd be able to convince my parents that these are symptoms of social anxiety - I know something is wrong with me, but I freak out when I try and talk about it and I forget what I was going to say. So I don't think I can get therapy.
So I've googled all the ways to beat social anxiety and there don't seem to be many ways to do it on your own - all the strategies that are suggested never help. Is there anything I can do or learn that will help me beat my social anxiety? Even small steps - what are the stages of beating social anxiety?

your post here is the first positive step.
There is a need to keep on making one by one: many steps.
Not every step will be step forward, since when learning how to handle life with anxiety, it is expected to trip over a hump and make some backward steps.
Social anxiety is more common than we think, where even "normal" people suffer from it.
I have to believe you have one person out of the house you can confide in and take at least short walk with.
It is advisable to walk outside during the daylight for 15-30 minutes in one or more attempts.
Contact with nature is necessary as well as contact with people.
If you planning on taking IPad and listening to it on the walk outside, don't do that!
Be aware and observing of your surrounding, dont let yourself become a victim of someone grabbing your earpieces along with it just because you wanted to sink into surreality by locking your senses to the music in your ears.
Talk to someone who you know out of your house. Talk to the school administration or your teacher. If your teacher is aware of your suffering, teacher will try to connect you with your parents on necessary level so they may find you a psychologist. If you are spending your day on internet, may I say, chances to heal from social anxiety are non-existent.
Internet is modern time disaster and is to blame for 65% of social anxiety these days.
Connecting with someone on Internet is not a connection is not a way to beat social anxiety. It is sparkling social anxiety and it needs to be limited. Young Family members of Bill Gate were not allowed to spend not even a two hours in 24 hours on computers: period. Forget the Internet, they had limited computer use.
Computers these days are meant and made useful with all the necessary gadgets for disabled people so they can work and earn for their living instead of exploiting welfare money.
The other day I was amazed seeing how quadriplegic person injured in car racing accident was using his computers to help me with my car needs and off it went my $950 to his company account. He was even extremely nice and honest since he quoted me initially with $1150.
I felt shameful that my brain, my two legs my two hands need 2 weeks to earn, and/or 5 months to save, that much money.
People with both hands to use, legs to walk, and with functioning brain are not supposed to rely on computer to live.
Reading, writing, coloring, drawing is still possible without computers.
Dear you, keep posting here in the brakes of you not being outside. Spring is here, take a book and notebook outside in the yard or in the park. Keep journaling anywhere you go and go back to read your journals. While journaling try to list everything that comes to your mind what's bugging you. After listing it think and write what you think may help you to not feel that way.
This PS community will help you great deal, but don't force yourself to climb Kilimanjaro before you reach its valley first.
If someone is claiming that something helped them and it should help you too, give it a try if it is free of charge, but don't panic if it doesn't work for you.
We all have the same goal in mind to "beat" the anxiety, but the maze pathways are unique to everyone.
Although every maze has an exit.