Unlike games, it has no quests to complete, no scoring, no leveling, and no end. There are certainly many games within SL, but SL itself isn't a game.
If anything, it might be compared to a chat room with a whole lot more added on. There are no goals other than those you set for yourself. You can get an education, socialize, get a job and even make real life (RL) money., and that is just scratching the surface. You can do pretty much do anything you can do in real life here, and a lot of things you can't do in RL. For example you can be pretty much anything here, from an animal to a plant. Your imaginations is the only limit.
That said you will find that SL is very much like RL. You will find some very nice people here, and some not so nice. Fortunately we have the ability to "Block" people that annoy us.
Also worth noting is that others cannot physically harm you,.
There is no way to cover all the different aspects of SL in a reasonably short space just as there is no way to cover all that can happen in RL. However here are some tips that can help you get off to a good start.
Learn your viewer. They all have a "?" in various parts of the interface. Clicking them will open a window with help on that part of the viewer.
Don't be afraid to click (and right click) on things you see as that is how you interact with objects in SL.
Read people's profiles. Quite often you can learn things about them and even find places, things and people that interest you. Also, fill out your own profile, let others know a bit about yourself.
Remember that not everything is what it appears to be. Just as that fox you spoke with yesterday is not a fox in RL, some people are not what they present themselves as. This is their second life and they may simply be in the persona they want.
SL consists of regions (sometimes incorrectly called sims) that may, or may not be connected. These regions may also be subdivided into parcels. Each of these may have their own rules. Quite often you can see the rules by going to the world menu at the top of your viewer and click on parcel details. The general tab will show a parcel description and the Covenant tab may show a covenant for the region. These are the written rules for an area. Just as different countries/states/provinces and territories have different laws so do regions and parcels.
Regions all have Maturity ratings. These are General (G) Moderate (M) and Adult (A). These ratings limit what the content can be as well as the behavior that is allowed. Please see the Maturity ratings link at the end of this notecard for more information on what each entails.
A fair number of people use voice chat to communicate and while you may not want to use a microphone yourself, having voice enabled will let you hear them and thus you won't miss the things said in voice.
Some important links to information you should be aware of:
Terms Of Service: https://www.lindenlab.com/tos
Community Standards: https://secondlife.com/corporate/cs.php
Maturity ratings: https://community.secondlife.com/knowledgebase/english/maturity-ratings-r52/Section_.1.2
Voice etiquette: http://wiki.secondlife.com/wiki/SLetiquette
Age requirements: http://wiki.secondlife.com/wiki/Linden_Lab_Official:Teens_in_Second_Life
Firestorm Gateway Covenant - Code of Conduct Rev. 2019-08-06: https://sites.google.com/view/secondliferatio/fs-code-of-conduct