Our first few days in Second Life were spent - along with everyone else that came in on the wave of immigration that followed a number of news stories - trying to figure things out. What were the rules? What, exactly, do you do here? Why is this freaking box on my head?
The short answers are:
1) the rules are whatever the community you're in decides it is. If you aren't sure, the same common courtesy that I hope drives your daily life will stand you in good stead here. There are combat areas and sex friendly (whatever variety you prefer) areas where you can get your ya-yas out, but by and large, be polite and you'll be treated in kind.
2) you can pretty much do whatever you like. There are casinos and red light districts, plenty of clubs where you can go dance with musical styles across the spectrum, games you can play (mostly varieties of bingo or Yatzee), sporting events like fishing, hop in a car and go for a drive, fly across the landscape and enjoy the view. There are parks, downtown cityscapes, and regions devoted to ancient Rome, the Victorian era, Japan, the far distant future, the Old West, and medieval Europe. And that's just the barest of beginnings. If you can't find what you want, you can buy land and create your own world. You could fill a whole book on the possibilities, and in fact, someone has.
3) Oh, honey. Right click the box and select "Detach". Then open up Search and look under Events with your Education filter on. Trust me - a little time listening to the wisdom of the teachers there for you in Second Life will save you hours of confusion and no small amount of finger pointing and laughing. (Not really - I've found people to be helpful far more often than not. But go take those classes. Seriously)
The only other thing I can think of that I'd have like to be able to say to my newbie self is that you don't have to spend money to enjoy Second Life. A good sense of adventure and the observant eye of a good tourist will keep you in thing to do for a long, long time. Freebies, some of excellent quality about in Help areas and frequently in stores, so that you can look good while you're touring across the grid. That said, I don't play it that way - I love to shop, I own land with Mickey and that costs money, too. None of the jobs available in SL for people who haven't yet learned to code or build are worth the investment in time to do them to me. If holding and SL job is entertaining to you, by all means, take one. But if it feels like work to you, remember you're making real-life pennies. I'd rather spend my real-life money to buy Linden dollars and spend my Second Life time playing.
At some point, I'll learn enough about creating things that I might try selling something, but I'm nowhere near there yet. I'm still a tourist (and landowner) and delighted by it.