when i try to a join a server it tells me i have a different class
can anyone help me?

You're trying to join a server which uses a different version. Such as playing Sourceforts 1.9.3, and trying to join a 1.9.2 server. (Vice Versa applies as well.)

So it could be you need to update to 1.9.3, or you could stop trying to join an old 1.9.2 server.

Also some 1.9.4 servers exist, so yeah you might be trying to join that as well.