I'm in 6th grade too! Never had a crush before though, here are some things I can tell you!
1. Maybe be friends with him. I do think it's better to wait until you are both older to have a relationship, but being friends first is a good way to start! You said your best friend's his sister, maybe she could help you start talking to him.
2. Your not ugly! Your just going through puberty and stuff, so I suppose I know why you would probably think that, but you are NOT ugly!
I'm not very helpful with these kinds of things, but I hope I helped, and remember to just be confident with yourself!