These are not listed in any particular order of importance.
These are both my own and paraphrased from a wonderful thread on the forums (see bottom of note for link). Thanks to all for such great advice.
1. Be kind to others and treat them with respect -- there are real people behind the avatars you encounter.
2. Do not "push" or engage others in combat (unless in a combat zone, and even then they have rules to follow).
3. IM someone if you'd like to tour inside their home. It is considered rude to "invade" someone's home even for the purpose of benign exploration. You can always click the main menu's World> About Land... and read the owner's [Description] -- their land might be open to the public.
Private sims are regions where the owner of the sim does not wish others to roam around their property. Just because you can, doesn't mean you should. Please respect their wishes and if you are intrigued by the look of the area, contact the owner (see above.) for a tour -- they'd likely be delighted. Most private sims or regions are a person's home, a specialized RP region etc.
Public sims are regions where the owner has opened up their lands for exploration to anyone. Even so, many have rules of conduct that you should adhere to, and if all else fails, treat others as you wish to be treated. Public sims are often learning centres, gardens, clubs, malls etc.
4. Always use a person's name when initiating discussion in Chat or referring to a specific individual's situation (i.e. Hi, Lucrezia! or Hey, Lucrezia, where did you get your kitty ears?).
5. Chat in public, IM when the other person is not in the vicinity. People will not hear you if there is more than 20 m distance between you (so if they don't respond in Chat, they might not have heard you).
You Chat by pressing the [Chat] button on the bottom of your screen if you do not see a white text region next to [History], then type into the text region and press <Enter>. Click [History] to view of a chat history (this is VERY handy).
You can IM someone in a variety of ways. You can right click an avatar and [Send IM]. You can IM someone you've friended by clicking Communicate> Friends> (friend name)> IM/Call. From a person's Profile, you can click [Instant Message...].
6. Not everyone has time to chat. Some people are building, scripting, sorting inventory items etc. Just move along and find someone else to chat with. Don't take it personally if someone is busy.
7. Don't rush into anything, including purchases.
8. Have fun! Try not to take anything too seriously, if possible. Remember, this is da intarwebs, and one big melting pot of an experiment. But please, ever be mindful of point 1. above.
9. Don't wander around naked -- wear underclothes if trying on demo skins. Never try and dupe people into the idea that you don't have clothes. You do... in your inventory... lots of them; and while not attractive, they're functional until you acquire better.
Click on the [Inventory] button on your lower screen. Look in the [Library] folder. At the very bottom there's a subfolder called [Clothing], and guess what? It can't be deleted, so you are never without clothes. :-)
10. You don't need to offer friendship to everyone you meet. Don't be upset if someone doesn't return the offer (it's not personal, it's just that most people try to limit their Friends list to those they will spend time with regularly). You can add a friend by bringing up their Profile and clicking on [Add Friend...]. You can also right click on a person and choose [Add Friend...].
11. Read people's profiles before chatting with them as it will often contain useful information (i.e. "I'm not interested in a relationship", "I'm just an alt account", "I'm shy" etc.).
You can view a person's Profile by right clicking an avatar and then [Profile...]. You can also right click an item, then Edit...> More>>> General, and then click [Profile...] next to [Owner] or [Creator]. You can view the Profile of someone you've friended by clicking Communicate> Friends> (friend name)> Profile.
12. There are no get rich quick schemes. Pay for an account, receive a stipend or buy Linden money. It's not worth camping for a couple dollars every hour. The best advice: learn a trade, do what you love (not what you THINK will earn you money), make friends, barter, have fun, and you might just make money by accident ;-)
If you try to scam or pull a fast one, you will eventually be found out and ostracized. It's not worth trying to pull scams in a tight knit community when your reputation is based on your behaviour (not just your appearance).
13. SL is a world, not a game. Explore, make friends, find yourself.
14. Be patient. Let SL find you -- it will if you let it. Take your time.
15. Avoid RL questions and discussions until you've chatted with someone for a time and a degree of trust has been established. Even then, most people in SL keep their RL separate -- respect this.
16. Don't leave items behind. If you "rez" something (i.e. drag an object from your inventory onto the ground), including product boxes, TAKE them back into your inventory (right click the objecet and [Take]) or delete (right click and [Delete]) them.
17. Don't beg for money, items, friends, sex etc. It's just, quite simply, unbecoming.
18. What is there to do? Build, script, dance, attend events, join groups, shop, explore... You can do anything. So the question is: What do YOU want to do?
19. Talk (chat or IM) to someone (like a neighbour) for dispute resolutions. Don't rush off to file an abuse report UNTIL you've attempted to reason with the other party (keep chat logs). There is, unfortunately, very little protection against being conned, manipulated or otherwise harmed. SL relies on its own citizens to work together in protecting and helping one another.
20. After teleporting somewhere, wait for the world around you to rez in before you begin walking or else you'll bump into someone else or a building -- very embarassing, as there are those who CAN see you even if you can't see them.
21. Bling. Most people don't like a tonne of bling on another avatar -- it's distracting and well... considered a little tacky. The odd object with bling is cool... trying to replicate the image of a 90s rap singer is just not cool in SL.
22. When in Chat or IM, type (without the quotation marks) "/me" before your "action" phrase. i.e. "/me dips her hand in the koi pond." will appear to others as "Lucrezia Lamont dips her hand in the koi pond."
23. Treat every person you meet according to their appearance and profile (their projected SL personae). RL attributes need not matter. RL information need not matter. If you choose to divulge RL information about yourself, fine, but it's not expected and you can't expect the same of others.
24. Some people abide by the "what happens in SL, stays in SL" motto. These people keep RL and SL quite separate. They may be happily married in RL with a great career and in SL they may be an escort, or even partnered. However, RL and SL will always remain separate for them.
25. Maintain perspective. L$1,000 is about the price of a cup of coffee in RL. If you get conned out of a couple hundred Linden Dollars, is it worth the aggravation to fight back further after you filed an Abuse Report?
EXCELLENT ADVICE FROM Jopsy Pendragon:
EXPLORE - SL is diverse... a bit heavy on the shopping aspect at the moment, but still full of many different things. If an area isn't to your tastes, keep looking!
EXAMINE - cool stuff when you find it. Look up the owner and drop them a nice IM saying you thought their stuff was cool and why. Positive feedback may not always get a reply, but it's a great step towards meeting people.
EXPERIMENT - there is much to learn. Tinker! Gestures, attachments, clothing, avatar appearances... you don't have to own land to write scripts for attachments!
ENGAGE - SL has a variety of 'cultures', with their own various rules and protocols, some friendly, some aggressive. Adapt your language and activities to suit the theme of the area you're visiting if you can, or find communities that are more your speed. Don't expect other people to drop everything and behave how you want them to.
EXPECT - surprises, both good and bad. This is an evolving world, things change, sometimes for the better, sometimes not. Be patient. Also, if you ever ask for something, and say "Because I NEED it!" ... you can 'expect' to be laughed at.
[more here: http://forums.secondlife.com/showthread.php?t=171147]
~ Lucrezia Lamont