I'm a libertarian. The defining characteristic of libertarianism is the respect for what are called "natural rights" -- those rights you enjoy simply by virtue of being a human being. The founders of the United States mention some of these rights: life, liberty, and pursuit of happiness are specifically mentioned in the Declaration of Independence. This was not meant by the founders to be a complete catalog of natural rights but rather some examples, since we have a multitude and almost limitless number of such rights.
Do you have a right to free association? To free speech? To self defense? Of course. That's why the Bill of Rights was written, to lay out some of the crucial natural rights so that there would be no question about them. But there are others as well. You have the right to eat or drink what you want (assuming it is not stolen from somebody else). You have a right to travel freely in or though public areas. The rights you enjoy are limited only by the equal rights of others to enjoy their rights.
To make things simple, there is really only one right that you need to keep in mind in order to understand libertarianism, and to respect other people's rights and protect your own. The important thing to remember is this: All rights derive from the right to property. That may sound counterintuitive, but bear with me a minute.
We'll start with a simple concept: You own yourself. Most people would not argue that point; after all, nobody else owns you, right? Assuming you are an adult and not bound by law in some way (such as in prison), you are free to do as you please within the confines of law and whatever morality you subscribe to.
So now you have your first piece of property, your body. You are free to exercise and eat right, or to lay on the couch and stuff your gullet with Cheetos. It's your choice, because you are a property owner from birth. You can easily see now that rights like freedom of speech and association arise from your self-ownership. If you move your body (property) over here, you associate with these people. If you make certain words with your mouth (property), you vocalize ideas. Your body ownership gives you a multitude of rights to use your body as you please, as long as you don't prevent others from doing the same.
If somebody locks your body in a cage and doesn't let it out, they are interfering with your property right to use your body as you see fit. If somebody forces you to work for them against your will you are a slave, and thus somebody else's property. So for your body property to actually mean anything, we'd have to stipulate that you also own your time, as well as your physical body. Likewise we'd also need to say that you own what you produce with your body and your time, the "fruits of your labor." If that were not the case, then somebody could just take what you produce and you are a slave again.
So let's say you use your body and your time to pick apples for a farmer, and he pays you by giving you ten apples. Somebody else later comes by and steals those apples by force, or cheats you out of them by saying he'll look after them while you go talk to the farmer, and then runs off with them. At this point the thief has taken the fruits of your labor for himself, and the time it took you to earn the apples was essentially used in the thief's service. For that time, you are a slave to the thief. This is why any act of force or fraud is an act of enslavement against another.
We now have all the elements in place for a full theory of libertarianism and natural rights. People own their own bodies, time, labor, and lawfully acquired property. Because of the entailed rights of ownership, people should be free to use this property as they see fit. This includes eating fattening foods, smoking, taking drugs, or anything else you or I might find repugnant. I don't get a vote in how you use your body property, any more than I can tell you what color car you have to buy. For me to do so would make you subject to my will, and thus to enslave you.
And that, my friends, is the essence of libertarianism. We each own ourselves, and none of us gets the ability to enslave another by forcing our will upon them, either their body or the extension of their body we call property. To do so would be as morally reprehensible as it would be to take up the taskmaster's whip and do it the old-fashioned way.
