The first question most consider when buying a kilt is what tartan to wear. Individual tartan patterns today typically have a great deal of significance. It was not always so. Up until the industrialization of the tartan weaving industry and the romanticisation of everything Scottish in the nineteenth century, tartans did not have names for the most part, and those that did were typically worn without much thought as to what it might be called in the tartan shop. People wore tartans they liked. Period. And you can feel free to do the same today. In fact, it's probably the most traditional thing you can do.
That being said, time does not stand still. Today, the tradition has evolved to the point where tartans are representative and symbolic. Tartans that represent clans are the most familiar, but tartans can also represent families, regiments, cities, states, events, businesses, individuals, occupations, you name it. When you wear a particular tartan, you are identifying yourself with whatever that tartan represents. Most people choose to wear a tartan that has some connection with their heritage. But the choice of which tartan to wear is entirely personal. There are no requirements to wear a tartan, no authorizations needed. You simply pick the tartan you want to wear, whatever your reason.
Most people, no doubt, choose to wear a tartan associated with their family. Typically, if the surname you bear has a tartan affiliated with it, this is the first choice. However, there is nothing wrong with wearing a tartan from your mother's side of the family, or your great-great grandmother, for that matter. Maybe you like that side of the family better and wish to honor them by wearing their tartan. Maybe you find your father's tartan unattractive. Maybe one tartan is simply more easily available than the other. There could be many reasons to consider, but it is entirely up to you.
Once you have selected your tartan, you may still find yourself faced with a variety of choices. You may find that your clan has an ancient, modern, and weathered tartan, as well as a dress and hunting version. Which is proper to wear? The short answer it, they all are. Just pick the one you like. Now here's the longer answer.
The ancient, modern, and weathered tartans of your clan are not different tartans. They are the same tartan, woven in different shades. You can have any tartan in the world woven in the ancient, modern, or weathered color schemes. The modern colors are dark and bold, and are the standard. The ancient colors are more faded, and represent what a piece of old, vegetable dyed tartan would look like after years of fading. Hence the name "ancient." The weathered colors are the same idea, taken to a further extreme. What might a tartan look like after being buried in a peat bog for 200 years? This is what the weathered colors are supposed to suggest. Think of them the same as "stone washed" jeans. They are made new to look old. Many people at first choose the ancient colors because they think they are more traditional, or that these were the colors their ancestors of old would have worn. Remember, the ancient colors represent faded cloth. A piece of new tartan, woven 200 years ago, would look more like our "modern" colors than anything else.
Dress tartans and hunting tartans are another matter. These names don't refer to color schemes. They actually refer to different tartan designs. The long and short of it is that hunting tartans have more green (or brown, or some other natural tone) and dress tartans have more white. The names do not reflect actual usage or restriction in wear. You can wear a hunting tartan to a formal occasion, and you can wear a dress tartan while hunting, for that matter. Dress tartans reflect an eighteenth century fashion for women's tartans to be white or cream based. But this does not mean that men cannot wear a dress tartan today, any more than they can't wear a white shirt or pants. Hunting tartans are given as an alternative if your clan tartan is predominantly red (or yellow, or some other bright color). Green clan tartans don't typically have hunting versions. In essence, they already are hunting tartans!
The important thing to remember if you find that your clan has six or seven tartans to choose from is that they all represent your clan. None is "wrong" or "right." The selection is entirely up to you and your preferences. Sometimes a clan that is particularly large (like MacDonald) will have various branches within that clan, and specific tartans to represent those branches. These are usually designation "of" somewhere (MacDonald of Glencoe, for example, or Campbell of Cawdor). If you are of that clan, but not necessarily from that particular branch, you might not want to wear a branch tartan. But even if you did, no one would put you in the stocks. Again, it's entirely up to you.