My ex's birthday party! help pronto!?

ok so me and my friend got invited to my ex's birthday party...hes really sweet but hes kinda a slob..so we decided maybe so like tonight we decided to go which is next saturday! and idk what to say to him when i see him! im so nervous cause i still have feelings for him but he has a girlfriend! and im nervous about the party cause theyre might be games like spin the bottle or truth or dare and imm soooooooooooo nervous! should i dress cute or not?? it is a boy girl party im aloud to flirt arnt i ? please help me!