Language
For me, learning Chinese is an essential part of spending time in China. It makes your life easier - taxi-drivers will know where you want to go, shop assistants will be able to help you find what you're looking for, and waiters will bring you pigs trotters less often.
It also makes your life a lot richer. Living in China while not speaking Chinese puts you in a very impoverished environment. You are left unable to talk to people beyond those few who speak English, you can't read newspapers beyond the China Daily or watch TV other than CCTV9. You can't eavesdrop or interrupt, complain or praise. You will spend your life in an incomprehensible world.
Being unable to speak Chinese is much like being a child - you won't understand what's going on a lot of the time, and you'll need your hand held in any but the most basic of situations.
Where you are will make a difference - if you are in Shanghai or Beijing, you could live quite happily without learning much more than your address and maybe a few favourite dishes. If you are in the middle of nowhere, you'll find yourself on a much steeper learning curve.
I don't have the space or inclination to go into great detail on learning Chinese here. What I will say is that it's not as difficult as it looks - the characters are only the written aspect of the language, one you can almost skip over if you don't have the time or mind to spend much time on them - enough to negotiate streets and menus will sufice.
The spoken language is much easier than you think. Pronunciation is different, but not impossible. Grammar is simple, and vocabulary logical.
Don't let the tones scare you. If you aren't too serious about the language, simply skip them - you'll sound wrong, but 9 times out of 10 you'll be understood (and eating a pig's trotter once every ten meals won't kill you). If you are serious about it, pay attention to the tones, but don't obssess over them - it's easy to spend so much time learning tones you forget to learn words, and it's the words that you communicate with.
If I had to choose the single most important point for a beginner to concentrate on (and having started the paragraph like that, I do) I would point at pronunciation. It's really not impossible, though there are a few sounds that will have your tongue in tears. It is different though, and if you don't get a good basis here, every word you learn will be misunderstood and that will destroy your motivation.
To get you started, I recommend this book and this website to get you started. Where you finish is up to you, of course. Also, if you have any questions about the Chinese language, then the Chinese Language and Culture Forums are a good place to ask them.