I couldn't agree more with Jim. Our tourist industry here in Scotland has promoted this fallacy, encouraging people to believe that clans were based solely on surnames. As Jim said, the clan your ancestors belonged to would have been decided by their location, not their surname. It's also worth bearing in mind that your ancestors may not have been part of ANY clan, as many areas of Scotland, particularly in the south and central belt, did not use the clan system at all. Another invention is linking a particular tartan to a clan. This custom (again commercial) only became popular in the 19th century, long after clans ceased to be a reality in Scotland.
The purpose of a clan was to work together for the common good, and to provide protection in numbers. This was only possible if the group was tight-knit, living in the same community. It stands to reason therefore that someone named Campbell for instance who was living in the north-east of Scotland would have no advantage in being part of the Campbell clan, which was located in Argyll in the west.
Another myth is the belief that people of the same surname are all related. When it became popular to use fixed surnames (and in certain areas of Scotland this was often as late as the 19th century), many people chose their occupation (Fisher, Taylor etc). Many others chose a descriptive name (White, Little etc). Others chose a description of where they lived (Hill, Burnside etc). So it's easy to see that there would have been totally unrelated people named Fisher (for example) living all around the coastal areas of Scotland. What purpose could possibly be served by them being in a "Fisher" clan ?
Hope this helps explain the reality M, but if you still want to go ahead and buy into what has became a big industry, then maybe Jim and I should keep quiet !