Friendship advice/new friends

The people that I thought were my friends apparently don't consider me as a friend anymore. They leave me out of plans and sometimes walk away from me to have a conversation. I just finished my first year of highschool and I feel like everyone has made their circle of friends and I'm left on the outside. Does anyone have any advice on making new friends/ joining a new friend group? I'm not terribly shy but because of being left out I'm nervous that I'm just going to be rejected again. Any advice would be appreciated. Thanks!