Struggling with Social Anxiety: How to Make Friends?

I very much would like to not have social anxiety. I don't understand why it's there and I have made no friends of my own. it's like anytime I try to talk something stops me. it's like maybe embarrassment or fear of rejection but its not a surface feeling. my surface feeling is just "I do not know how to contribute to this conversation" I have no clue how to get a conversation started or how to keep it going on. I don't have the "common" knowledge to do so. what else contributes to this is that I also have slow processing disorder, selective mutism, and I don't really retain information very well. everyone thinks I'm rude or shy or an introvert. but I want friemds and to go out and be spontaneous and friendly to everyone. does anyone else feel like an extrovert stuck in the body of an introvert? Like I'm an extrovert that jist doesn't know how.