How do i act around my ex boyfriend and his new girlfriend?

my best friend is getting married in august and i am in the wedding. im super excited because all of my friends and family will be there and its going to be a great party. however my ex boyfriend is also going to be there. we broke up in January after he dumped me for another girl. he is now dating that girl and bringing her to the wedding! im so nervous. i dont know how to act around them or what i should do. im single and i dont really want to bring a date to the wedding but i almost feel like i have to. how should i act? anyone have any advice?