Threesome advice???

Hi guys! Sooo this guy I've been seeing casually invited me to a threesome with him and one of his friends. I said yes because it's something I've always wanted to try (yolo lol) but I'm really nervous!! His friend is female and I've never had a sexual experience with another woman before. I'm also afraid of feeling a little selfconcious/jealous if I see him with someone else? Does anyone have any hot tipz lol? On how to make sure I'm cool emotionally but also "pulling my weight" haha?