Hey, i just want to ask why do some people have [---] before their username? How can i do that?

Just write it into your login and it will change but you cant change your name

that space is for your clan tag

when you log in write Org[Clan]4venge and ingame you will have your account named org[clan]4venge

also int he clan position DO NOT use the tag of an oficial clan, you will be kicked out while you play

also org = organization

um...... so i cant just write anything i want in the brackets?

Yes, although it is usually a clan.

Would be good if you join a clan before you do that ^^
If you are not a member of a clan you don't need that tag.

You can also do so for organizations.

Ok, but Why doesnt everyone have a "tag", they could write anything.

Because it is generally only used for clans and organizations, you CAN right anything. But you'd look a little dumb. So there's not real point other than that.

Let me explain this:

You can enter a clan through the clan board, like-, on the first page scroll down till you see a board called clan discussion, there you can choose a clan and apply to join that clan. You should only apply to an official clan when you are a black belt, because thats the minimum of the most official clans.
There are also non official clans, those are normal threads started with [DSC]clanname. Its easier to join the non-official clans.
After joinning a clan, you can use the clan tag.