I live with my mom with my 3 siblings. She has full custody. My dad is getting married and moving to California. I want to go with him. He knows that and says its OK if my mom says its OK. If my mom says no, what happens? Do I go to court? Do I easily loose if I don't have a ligitiment reason to go? What if its just what I want? Do they put that into consideration? Do my siblings have to get involved in court and stuff? If they get along with her, and I don't, is that put into consideration?

It could all depend on your age. How old are you?

I'm 15.

More than likely, your Dad would have to go to court. The judge would take in to count that you are 15 and really want to live with him. But, that is not a guarantee that you would get your way.
It would be better for everyone if you could sit down, together, and talk this out.
Good Luck.