If you're looking to start a clan, you'll need to talk to the Scribe at the Clan Camp south of Falador to get started. Ask him about starting a clan, and he'll give you a Charter that you need to get signed by four other players, as it requires five players to found a clan. I would recommend reading this Wiki article about the Clan Camp because it has a lot of useful information for starting out:
Once your clan has been founded, you should familiarize yourself with your Citadel. It provides useful content for your clan, such as an XP boost from the Avatar habitat. Here's another Wiki article for Citadels:
There's a very helpful thread in this forum section titled Clan Help that I highly recommend using as a reference if you have any questions about running and maintaining a clan, or just general questions about how clans work. You can view it by clicking here: ? Clan Help ¿
Let me know if you have any questions.
Best of luck with getting your clan started!~ Sicariu