how do i get that writing next to my name like i see pepole with [Jayg] with there username next to it

You mean the custom text under the username?If you mean that, to change it, you need to buy Toriprime which costs $10 for a year.To see all the benefits of buying it, click here.

I bet he Mean Clan tags

i think there called clan tags.. there shown in game

You need to be in a Official Clan .
Here a List of Official Clans
http://forums.toribash.com/clan_list.php

Then if you login you wear The thag of the clan