Welcome to graduate school! I hope that you will be both happy and successful here. One thing that will increase your chance of happiness and success is understanding just what it is that you've gotten yourself into!
If many people you know—family or close friends—have been to grad school in the sciences, you may know all about it and may not need to read this guide. But if you're the first person in your family to go to grad school at all, or if the people you know went to med school or law school or humanities grad school, then I may be able to clarify some of grad school's underlying assumptions. Usually, no one tells undergrads these things, but everyone assumes that grad students all know them. So when were you supposed to learn them?
Usually people end up in grad school because they've done well as an undergrad. You're smart, you're good at academic work, and you think that grad school might be for you. That's a start, but it takes more than that to be a successful graduate student. Unfortunately, people often assume that grad school will be a lot like college, maybe with more advanced material and harder courses, but basically the same. This is a reasonable assumption, but unfortunately, it's wrong. Grad school is really very different from undergraduate school.
In grad school in the sciences, you are being trained to do one thing: to produce large amounts of high-quality research. (Not "research" in the sense that undergrads sometimes use it, to mean "going to the library and looking up stuff to put in a term paper," but "research" in the sense of "doing basic science to uncover new facts or new applications of known facts.") So if you're here in graduate school, it should be because you want to spend most of the next four or five years doing research and want to get a job doing mostly research, either as a professor at a university or as a staff researcher for a corporate lab or thinktank.
If you thought that grad school would be a lot like undergrad, you may be wondering about classes. You will take some classes (and probably teach some, as well), but in grad school, classes exist either to give you the foundation knowledge and skills that you need for doing research or for stimulating your ideas about research in a specific area. Classes are not the point of grad school, and if all you do in grad school is get A's in courses, you'll be considered a very bad graduate student. To be a good graduate student, you must—you guessed it—do research!
So how do you get started doing research? Maybe you've never done research before and are a little uncertain about how to go about it. Maybe you've done research with a professor while an undergrad but were mostly guided by him or her. Maybe you've done research for years under someone else's direction and want to be able to direct your own research. Maybe you came in with some great ideas for research but don't know how to work the system to get them implemented.
No matter where you are in this continuum, from research virgin to seasoned researcher, the way the system works in graduate school is that you are considered an apprentice of one or more of the professors. You pick a person to do research with, and that person figures out where you are in terms of existing research skill and helps you to become a skilled researcher. Usually you start out doing research that the professor has already in progress, gradually adding your own ideas and input as time goes by. By the time you do your doctoral dissertation, the ideas will be mostly your own, although you will still get your professor's guidance.
(When I say that you are an apprentice of your major professor, "apprentice" is not a cute little metaphor—it's a fairly accurate description of the life of a graduate student. If you were an apprentice to a blacksmith, you might start out life by keeping the fire hot and sleeping on the floor, never actually touching the iron until years later. Although we don't make you sleep on the floor :-), first-year graduate students do tend to do much of the lower-level research jobs. As you learn more about how research is done, you will gradually assume more and more responsibility and contribute more and more of your own ideas. Next year, there will be a new batch of first-year grad students who will take over your job of keeping the fire hot. :-) If you have an unusually nice major professor, the scutwork may be kept to a minimum. If you have an unusually exploitative major professor, you may find yourself doing large amounts of scutwork for large amounts of time. If the former, rejoice—you lucked out. If the latter, change professors if at all possible.)
All of this means that deciding with whom to do research is a very important choice! Most graduate students do the bulk of their research with a single professor, although many do some small projects with other professors. Because you will be doing research that was begun at least in part by the professor, it is important that you choose someone who is working on topics that are interesting to you. Because you will learn to do research at least somewhat in that person's style, it is important that you choose someone who you believe does high-quality research. How they treat you, as a student and as a person, also matters a great deal. You will be spending a LOT of time with this person, and they will have an enormous impact on your life!
When you are ready to leave graduate school, it is your major professor who will write letters of recommendation that will help to get you interviewed. It is this person who will call his or her friends and colleagues in the field and say, "I have a student who's graduating this year who's dynamite; you should take a look at this person." This is less important if you want to get a job as a researcher for a corporation, but if you want to be employed as a professor at a big-name institution, then your major professor's recommendations and contacts are vital.
All of this means that you should choose a professor who is willing to look out for you, who goes to at least a couple of conferences a year, and who is well regarded by people in the field that you hope to get a job in.
What will your major professor expect from you in return? Well, as you've guessed by now, he or she will expect that you'll do a lot of research. :-) And if you want the person to tell future employers that you're dynamite, then you have to BE dynamite! They usually expect you to work on their own ideas for the first year or two and to help write drafts of the papers that will be written up about the research that the two of you do together. Graduate school is not usually a 9-to-5 endeavor, so if a major conference has a paper deadline at 8 a.m. on Monday, expect to be working all weekend. (Yep, there are paper deadlines even at the professorial level!)
What happens if you don't really want to do research? I wish there were a gentle way to say this, but there really isn't—if you don't want to do research, then you're in the wrong place. Drop out before you waste a lot of time and money doing something that's wrong for you.
What happens if you live, breathe, and eat research? Then you're in the right place—everybody else here is crazy in the same way that you are. Have fun!