First, let's talk about the Game-play. This is a very hard topic to explain as what you do in Second Life depends entirely on you. If you want to shoot zombies, you can. If you want to go to a club and dance your heart out, you can. Explore haunted houses, watch a movie, ride a rollercoaster, you can! The game is basically one giant sand-box. Meaning, everything in the game is made, coded, and ran by players (except the game itself of course).
Keep in mind that this game is aimless. There is no set goal for you to achieve here but much fun to be had. To go along with this the world isn't connected. Instead of being on one giant land mass you're divided into regions called "Sims". Each Sim is different and its contents are based on who owns it.
Confused? Well, let me try to explain it. There's a button called "Build" that lets you start creating (you can't build in most places as to avoid a spam of objects, press ctrl+F, click the places tab, and type in sand-box. Find one you like and teleport there. These are the designated building sites or get more Second Life Linden).
So, when this build menu comes up you can choose shapes and if you click the ground it creates itself there. Now, if you right-click whatever shape you made you can edit it in many different ways. But if you click the content tab you will notice you can put or make scripts in there to make it to a wide array of things. You're probably thinking, "Hey, if all I have are these shapes then everything in this game must be all blocky!" Don't fret, I'll discuss that when we get to the graphics section. Also, since you can build anything you can also look like anything. The customization on here is endless, you can even be Sonic the Hedgehog!