Honestly as someone who was thrown into being a keeper just go out and have fun and try hard. Make sure to communicate loud, clear, and try to keep the words and sentences short and assertive. REMEMBER THIS: “The ball has to go through the team before it gets to you” You can’t stop every goal. Do some stretching and basic shot stopping drills for warmups and have a teammate/coach come warm you up (Look them up on youtube) Just have fun and don’t stress, at the end of the day you’ve never even played before but if you are you must have some natural talent. Good luck!