If your surname is the same as the clan name, it's pretty straight-forward. If not, it gets a bit more complicated and opinions differ.

The term "clan" and "family" are often used interchangeably (though some will say that it's lowland families and highland clans, the idea is the same), and a clan is essentially just a large and very extended family. So large, in fact, that some of its members may spell their last names differently; though they almost all sprang from the same stock.

Clans generally have a number of associated surnames (generally called "septs", though the use of the term is not universal); these can represent alternate spellings of the name or the names of families with different surnames that have historically offered their fealty to the chief of the clan. My advice is to be careful with such lists; if you find your surname on one, do your research - some names are claimed by more than one clan. I have family that was told by a storekeeper that we were members of one clan because his list said we were. His list was provided by his supplier, though, and it turns out my family would have been historically associated with another clan. I only discovered this after a decent amount of money was spent on the wrong clan's paraphernalia.

Clan allegiance was also largely geographical; if you lived in a chief's lands, you were his vassal and would generally show loyalty to his clan. This meant that some families "changed hands" over the course of history, and that many clans have members with trade names (such as Smith) since each clan needed its tradesmen to shoe the horses, make barrels, etc. Where your ancestors lived is a very good indicator of the clan to which they were loyal.

So, after all that, I'd suggest you start with your surname. If you can track your ancestors on that line back to Scotland (and there are loads of websites that can get you started for free, and others that you can probably use through your local library), then that should give you an idea of the clan to which you would belong. If your surname isn't Scottish, start looking at other branches. It sounds like you're related to the MacPhersons in some way, so that would be a perfectly acceptable route to take - just keep in mind that traditionally clan affiliation is based on your last name.

I hope that made some sense! Good luck!