see the clan info (http://www.lineage2.com/Knowledge/clans.html) on this page, once you start a clan, speak to the head of a Warehouse or church, or even a head trainer and click the clan info option... you can see the SP/adena requirements there, and the NPC to start your quests. i would list them here but ive forgotten, sorry! :p
There are costs involved. I think at one point you need to pay something like 2 million adena but after that the costs are in skill points.
What we did was take donations from the clan members and I just filled in the rest.
But there can be other costs as well. We had a program where the clan would give each member a trade of C grade armor for their D grade armor (or robes). I ran my dwarf in order to fund that program. Of course this was back in the day where people when people levled slower so it was never more than 2 at a time. Usually just one.
What I'm saying is that as a clan leader you will want to figure out ways to help your good, longtime members. Could be clan hunts where all the adena and drops go to clan resources, or it oculd be where the clan gets together to make a weapon and you sell the weapon for profit for clan activities and programs.
I didn't realize that the requirements for each clan level were not listed on the official web site, so they have been added to the Clans (http://www.lineage2.com/Knowledge/clans.html) page. Thanks for pointing this out!
Noticed following paragraph missing information about the Goddard clan halls though:
Clan Halls
Only leaders of a level two or higher clan without a clan hall can obtain one. Clan halls are available in Dion (C Grade), Gludio and Gludin (B Grade), and Aden (A Grade). Each hall has been given its own name; for instance, the first house in Gludin Village has been given the name Gludin Crystal Hall.
Plus you may add that the owners of Aden clan halls may now use a Wyvern equal to the castle owners.