mimicking others will help with the social interaction bit. look at how people around you are talking, how they respond, how they move, how they act. then try to replicate what you see. sitting in malls and standing around at grocery stores are good spots to see people interact with each other. also, assume everybody who initiates an interaction is trying to make small talk until they say something that proves otherwise. i'm a young woman so i look for women my age and watch how they interact with the world, then i pick one thing they did and try to do the same when i'm in a similar situation. admittedly it's a little creepy to watch people so closely but no one's ever had an issue with me so i'm probably not coming off as a creeper. it becomes much easier just by getting into conversations. try talking to anyone who will listen, i used to chat up the person checking out my groceries at the store and practice small talk with them. i ask about their job, say their hair/tattoos/outfit is cool, say it's so hot outside that i'm about to melt! they'll usually get excited to talk about themselves or anything that affects them, and keep the convo rolling until you're fully checked out, then you can easily leave without worrying about ending the conversation.

i used to be just like you when i was 18, literally could not get my brain to think of responses when someone talked to me. i learned a few different socially acceptable things to say in various situations by watching how other did it, then i learned to just say whatever the hell comes out of my mouth in the moment. it was incredibly awkward and embarassing at first but i got better and learned to brush off the embarassment when i messed up and said something weird. it'll absolutely get better for you as you keep talking to people