If you cannot trace your ancestry, the custom is to pick one of the Clans that list your surname as a sept. Here is an article that explains the custom:

http://www.electricscotland.com/webc...lies_septs.htm

The Clan Muir/Moore is another Clan that has a lot of sept problems check out the wiki article here:

http://en.wikipedia.org/wiki/Clan_Muir

Good luck.