There is a guide on the offical site here. (http://www.lineage2.com/Knowledge/clans.html) The 'NPC responsible for clan creation' I think is the highest dude in the hall where you train skills. Look for someone with an important-sounding title, like 'High Tetrarch So-and-So.' When you talk to the right one, you get a 'Clan Creation' option on the dialog screen.
Just wanted to make sure you also knew that creating a clan and raising its level requires lots of SP. If you are asking questions about creating a clan I will assume your a relativly new player. New toons normally are very low on SP, so make sure you have plenty to go around before wasting to much on creating/leveling up a clan. Oh yeah, and it cost adena to level it up too.
Thanks very much for the answers guys..
I 've been actually playing since november 2005, so i m not exactly a nooby but I've never looked into this at all :)
Keep in mind, if you decide to dissolve your clan, it'll cost you a death worth of xp. -If you decide you don't like the clan name, and dissolves, once it's dissolved, you can't make another clan for another 10 days - after it's dissolved. Yeah, I learned that the hard way :p