First I have a member in my club who is not being very nice. We invented the club and we have a 50/50 share of it. But he wants to have the club all to himself.

So he comes to school nearly every day with some kind of new unofficial letter saying 'I’m the leader' on it. And then he calls me a loser and says he fired me (though leaders can't fire each other (it says in the rules we agreed on) then he says he will promote someone to take my place.

I have always stuck up for him but he isn’t very nice back (sometimes) what shall I do?

Is being in this "club" so important that you're willing to put up with this drama? Why not just leave and make another club. Invite all the people who aren't acting like children.

By 50/50 share I hope you don't mean you have money involved in it. If you do then it would be your own fault for not making a contract to uphold your "leader" status.

Cut your loses and walk away from this.