Clans nowadays are mainly common interest groups with occasional get-togethers and usually a bit of genealogy. I have to say also that they are more of a trans-atlantic phenomenon than something that most mainstream Scots get involved with. They do not, however, hold any exclusive rights to a particular tartan so you should not prevent yourself from choosing one if it is to your taste.
Also simply because you may have a Scottish-sounding name is no guarantee that you actually belong to the clan of that name. Whatever your name, your clan affiliation would have come from having ancestors who lived in a particular place and owed allegiance to the clan chief there. You may well, therefore, be called MacDonald but if your ancestors lived in Inverary (and this is hypothetical remember) then you would most likely be a member of Clan Campbell. Or they could even have been slaves with a Scottish master (sadly this was often the case) who took on their owner's name which if it happened to be Campbell, wouldn't then make them members of Clan Campbell.
So please take a lot of these clan and tartan rules with a generous pinch of salt and make your own mind up which one to wear.