To be honest, I would wait until you are a little bit older; but if you're deadset on doing it, there are quite a few things to know and keep in mind. A few other posters have touched on a few of these subjects.
First of all, this is not an experience where one just tries to get "fucked up". It's not remotely like marijuana, and these little fungi are VERY powerful. Although some use them recreationally, they were never originally used that way, and all of the people "in the know", including psychedelic researchers, psychonauts, shamans, and everyone else strongly recommend against using them just to get intoxicated. There are quite a few reasons for this, but primarily this is one of the reasons people have "bad trips". They don't realize the how extensive of an effect these little mushies will have on your brain, your sense of self, and even the very ideas you hold to be true. Prior to the 1960's (and even more so, the 1980's), these were used primarily for spiritual purposes. That's why they go under the label "entheogen", which is basically Greek for "God [born] within".
But, if you're looking for a real experience, don't let that scare you away. There's no concensus on the appropriate age for people to take entheogens. Just like an asshole, everyone has an opinion, including the most knowledgable people. The only reason I caution (but don't deter) you from taking them this young is that however mature you think you might be, your ego (or sense of "self") isn't as fully established as someone who's entered into adulthood. And the magic thing that entheogens do is that they fragment your ego, or even obliterate it (called "ego death"). That's why people smoking Salvia think that they are in fact paint drying on the wall, some people on LSD have difficulty distinguishing themself from someone else, etcetera. You become very attatched to your sense of "self" and it can be frightening when it's being torn away from you. People often start obsessively contemplating their own mortality in more direct way than they ever have before, and some people even convince themselves they're probably dying when it's coupled with the paranoia the drugs produce.
But, there's nothing to fear if you just give into it. Dive right in, push away your fear, and do so in a comfortable environment with people you implicitly trust (called "set and setting"). The other posters are right. You will feel uncomfortable around people who aren't tripping, because they can't possibly comprehend your state of mind. But sober trip-sitters keep you safe. All of this paranoia and fear is caused by the drug's effects on your parasympathetic nervous system. This control multiple functions of your body and psyche, among them, urination and fear. Every little sensation will effect you in some way. You'll probably see hallucinations, and they will probably amuse you. But the amazing effects are the ones it has on your mind and your thoughts. It doesn't just cause fear... it also causes you to evaluate your life, your actions, your connections to others, sense of self, sense of God and your perception of the world around you in general. And if you go with it, it's not a bad thing at ALL. It can be one of the single best experiences of your life, next to say... bringing a child into the world. And no, I'm not kidding. And that little ego-death thing I mentioned before? That's the goal of a lot of psychonauts... To see the world without any preconceived notions and realize that "who" you are is not "what" you are. People often compare it to nirvana and a sense of "enlightenment" and even being "one with God" in certain religious traditions. It's pure consciousness.
But, those kinds of effects mainly come from what's called a "heroic dose". More accurately, with natural hallucinogens, it's somewhere near the original dose that people used them for when taking them for spiritual purposes. In the case of psilocybe cubensis mushrooms, that would be the Aztecs, Mayans, Mixtecs (all basically in Central America), certain South American peoples, as well as Tibetan and Thai Buddhists in rural areas. These people still ingest them at these doses for these purposes. A "Heroic Dose" for shrooms is roughly 5-10 grams dry. Most people recommend seven grams. This makes the peak approach quicker, last longer, and everything is more intense. Except the fear, really. It goes away pretty quickly when you plunge right into the void. But keep in mind, at much higher doses, you might eventually loose the ability to understand and use language, understand basic concepts, and interact with the world around you in any normal way. You're on a completely different plane of existence, and things like "feelings" and "concepts" don't really matter much anymore.
If you're going to eat a half an eighth, or even an eighth, you're not going to get the indescribable spiritual effects from it. And for first timers, that's almost always a good thing. You'll probably get deeply introspective though, and you'll probably also be giggly, confused, have flushed skin, have some stomach cramps, and probably some nausea (also your sympathetic nervous system, not just the shrooms themselves). Along with some trippy visuals. You'll probably also want to reform your life in some way the next day (pretty common... people sometimes want to even give up any drugs for a while because they want to just "live"). But chances are, if you're where you're comfortable, you'll have a good time, and get something out of the experience. Just remember that wherever the experience starts to lead, don't fight it. Follow it, and just go with the flow. Putting up a fight to the effects of the shrooms, rather than just accepting them, will likely result in a bad trip. If you're not outside in public, and you don't do anything stupid yourself, you're not going to die (no one ever does from just taking entheogens, despite what you've been told), and you're not going to get caught. Just thought I would try to be a little bit more detailed and give you an idea of what these things are really "supposed" to do, and why they have the effects they do as a result.
Remember, don't do them if you just want to get fucked up. But if you want a genuine EXPERIENCE, whatever that may turn out to be, go for it. But remember to be safe, and be positive. Also, smoking a little bit of bud close to the peak (when the effect get their strongest) will intensify the experience, and probably mellow you out a bit. Also helps with the nausea.
Hope this helps.
~Will