When you begin working on your PhD, you will continue working on your PhD until it is complete, at which time you will either directly enter industry or a postdoc. A gap in a resume is a serious concern to many employers, unfortunately. If you do a postdoc, you will continue doing postdocs until you get a job and then you will work for the rest of your life, with bills to pay and mouths to feed. A three-month vacation is not in your forecast at any other time in your life other than after you retire. Now would be a good time, not just to vacation, but to have the best vacation of your life. Go where you've always wanted to go with someone you'll have a great time with. The probability that you will ever have another opportunity like this is slim to nil. And go ahead and spend some money. It's okay. You won't be as poor being a grad student as you were when you were an undergrad.
That doesn't mean some preparation won't help. First of all, you should try to think about what kind of biologist you want to be. You should try to be good at it, whatever it is. It's going to require some planning and introspection. One thing to anticipate is that you're going to be very busy and under a lot of pressure. You should plan for ways to deal with that in advance. One way of dealing is to have some hobby or something as an occasional escape. Be careful though in choosing your hobby. Try to choose something that doesn't take much time, and that isn't intellectually taxing. My hobby was learning Japanese, a bad choice on both grounds. If I was to do it over, I would try a sport or something physical. That gets your mind off your work, and it can release a lot of stress. Another thing to prepare for is planning time for your own personal study. You're going to be terribly busy studying for exams and doing homework, etc. Find something that you're interested in, and look into that. Figure something out on your own that is not required work. This actually helped me a lot at the times when I felt overwhelmed. Another thing you're going to want to work into your schedule is some career planning. Do something once every few weeks. Look at indeed.com, craigslist, jobiology or whatever, and see what companies are looking for. As bad as it sounds for biologists, as I'm looking for jobs, there are many more options there than for physicists. If you're reading slashdot, you likely have some technical abilities. Programming often comes in handy. In physics, some programming was used in about 80% of PhD's, and I'd guess it's somewhere around 60-70% of biologists, but don't quote me on that. Anyway, if you like programming, it couldn't hurt to get a little more experience here. Scientific programming is different from sys-admin or soft-dev, so maybe look into some of the well-known programs in your field and get to know them. Chances are, these days, a lot of them will be open-source. At least many in physics are. Personally, I think it's a good idea to program in a language that is common in your field. If it's Fortran, then it's Fortran (it's not as bad as it used to be). But go with the group on this so you can cooperate with your colleagues.
In your PhD, the three keys to success are not location location location, it's Adviser Adviser Adviser. This is one of the big choices you will make in your life. Do it carefully. Your adviser will be your parent, prosecutor, parole officer, and savior (or destructor), all in one. There are some very excellent advisers out there and there are some real assholes too, and it's actually kind of hard to tell even after being there fore a couple of years. Other commentors have mentioned some useful ideas for selecting an adviser, so see those too. It's sometimes hard to get an honest comment from current students of some advisers because of the conflict of interest, so really ask around and talk through your ideas with some of the older grad students, but definitely more than one. One other point I'd like to make is that you should select someone as good/prestigious as possible. I opted not to go with the best because I saw how many students he had and I figured he would be too busy, but it's not the quantity of interaction you have with your adviser, it's the quality. When you get going on your project, and really get into the details, you will know more about it than your adviser, probably, and anyway you will be doing 99% of the work all by your lonesome. What can really make or break you is that 1% of the time and whether your adviser can help you out at that point. Also, better advisers are way better at lining you up with a good project, something that is publishable, important, and doable within a reasonable time span. Crappy advisers are more gamblers, will throw you at what looks like promising projects, but turn out to be fads or total time-sinks or just plain shite. And they won't be helping you out with your career, you will be padding their resumes with your publications. Oh yeah, I forgot to mention, professors get raises when you publish more, so they may push you to publish earlier than is reasonable. You will have to walk a fine line between doing questionable work and not pissing off your adviser. Unless you get a good one, then you don't have to worry so much. But they need publications too. Anyway, if you realize you've got a crappy adviser, switch if it's early enough, or just stick with it if you can. I finally told my adviser to fo and now I'm in a bad way. Anyway, as a bit of personal advice, just as you shouldn't date a co-worker, you shouldn't try to be friends with your adviser. Keep it professional. But then again, you're kind of stuck with whatever they want the relationship to be. I guess just roll with the punches.
Graduate courses are weird. They're much harder than undergrad courses, but then again, in some ways, they're about the same. I don't know. One thing you may realize is that the sky is the limit, that there are an infinite number of things you feel you should study, and you just don't have the capacity. It's crazy, probably none of the other grads will agree, but my advice is to compare yourself with the other grad students. If you're understanding things about as good or better than most, then you're okay. You don't need to be a genius. It's supposed to be difficult. That's why you're there. It's hard for geniuses too. The first two years of courses were the hardest, but they were also the time when I felt the most capable. You're going to feel overwhelmed and like you don't understand anything, but when you're done you're going to look back and say, "I know kung fu!" or whatever it is you know, and you can be proud of that.
Some of the other commenters have given you some decent advice. The PhD is a serious journey with a lot of cool stuff and a lot of total shite all the way through. It's also a gamble. First off, many will flunk out or simply quit grad school. Many flunk on preliminary exams or in qualifying exams, but some quit because of crap projects or advisers. And you hear people with the PhD's complaining about the job market! Next, for some reason, we pay doctors, lawyers, MBA's and bankers exhorbitant sums of cash, but scientists get little pecuniary respect, generally speaking. When you begin grad school, you will probably be a T.A. for which you will be paid, on top of having your tuition taken care of, and you'll think, oh snap, this is a sweet deal! But you have to realize that the $20k or so per year you're getting is only a fraction of what your engineering pals are already getting paid right out of college, or what your construction buds are getting with no real education whatsoever. If you're lucky it will take 5 years. I think one or two out of forty in my class graduated in 5. The average is between six and seven, but many also take eight and some nine years. So at $20k/year for 6 or seven years, you'll basically be working for room and board, even though you're already one of the most highly skilled, highly trained people around in an area that's critical to the U.S.'s economic growth. And then things really get dicey after you've spent all that time getting superior training. I'm not saying don't do it. But I'm telling you, honestly, it's a gamble. And these are some of your best years, when you are setting the course of your career. There is some real risk here. I don't want to dissuade you from the PhD. I think it's probably a good option if that's what you want to do, but I don't want you to go in there blind. Your school is your main source of info on your career at this point, but they really do keep you in the dark on a lot of things.
Well, there's a lot more I could say, but it's getting late and you've probably stopped reading a long time ago, so I'll stop here. Wow, looking back, it sounds fairly pessimistic. I guess I was just one of those for whom the gamble didn't pay off. I'm a better physicist than 90% of PhD-holders, but got a crappy adviser and crap project. If I had the opportunity to go back, would I do it again? Yeah, I'm pretty sure. I'd just choose another adviser. There really is a lot of cool stuff that happens as a PhD student, etc. Good luck to you, whatever you do.
To summarize, be smart, stay tough, deal with the stress, and strive to be a master at what you do. There is no spoon.