How does banning from a web work?

They ban your IP address. People usually log in from a specific location and that address can be easily read -- and banned -- by a server.

Re
can i somehow changed my ip or sever?

Re: Re

Yes - well enough to work, anyhow.

Why were you banned?

I think you would have to change internet providers.

most ISP's work on a dynamic IP scheme. unless you are in a dorm at a college, then you most likely will have a static ip. but if you are using a cable modem or DSL modem, you could try to turn it off for a few minutes. it will be assigned a new IP after restart. however, there is the possibility that it will be assigned the same IP. if that is the case, call your provider and ask for a new ip address.

As an administrator on several internet forums I can tell you that there are usually very good reasons for banning people.
Instead of changing your ISP or learning everything there is to know about IP hacking I suggest you write to the administrators of the forum and politely and respectfully apologise for what got you banned.
Then leave.
If they invite you back then all is well and good. Just remember you can be banned again just as easily so toe the line this time.
If not then take your punishment and regard it as a saultory lesson.