If you are unaware of what Second Life is, then here is a good source of information; another fine place to look is here. This is not strictly an environment limited to Linux users; Windows and Mac OS X users are also able to run the software to enter the environment.
This is not a tutorial on how to run Second Life under Ubuntu; if you want one of those, head on over to Ubuntuforums. This is more of an anti-tutorial (or whatever the opposite of tutorial is--a how not-to?), and what you should avoid doing, i.e., do the exact reverse of what is chronicled here, or something.
If you are using Ubuntu Linux as your computing platform of choice, then the first thing you want to do is head over to the Second Life web site. There you can read about what is Second Life, join up to the environment (all that is really needed for this a valid email), though first it might be a wise idea to read the frequently asked questions, or as is otherwise known, the FAQ. Which is available here.
Of particular interest to Linux users will be the system requirements, a list which can be found here. At the bottom of that page is a list of what video cards do and do not work with Second Life; if you are a Linux user, you can pretty much ignore that and game environment on regardless. Why that is, I have no idea. One absolute must is a DSL or cable internet connection--no fudging on that one.
Once you have ascertained whether you have a DSL or Cable internet connection, then you should head over to the downloads page, and um, download the Alpha client for Linux. Wherever you download it to, you need to extract the file from its compression; for Ubuntu users, simply right click with your mouse on the file and choose 'extract here'. The files in the compressed package will then be extracted into a folder, and you are almost ready to go. Just two more steps.
Next up, head over to second life's home page, and join up. You should then check your email, and activate your account. You can then quit your email program, or web browser, as you will be entering the game through the special viewer you downloaded just a moment ago.
Back to the folder that you just extracted from its compressed state; you now need to either double click on the file named secondlife.sh (haven't tried this, but is said to work), or alternately, open a console window, and type the command
./secondlife
followed by your password. This will launch the second life viewer, and there you will enter your chosen name (when you signed up, that one), and your password (also at sign up time).
The viewer will take a moment to load, and then you will be asked to choose your avatar, or virtual representative in this virtual world. This can take quite a long time, depending on how fussy you are in regards to the little details, or no time at all, if you just want to go with the stock out of the factory look. You can change your appearance at any point during your time in the environment, so no need to worry about it either way.
The stuff before this was what you should do, from now on out, don't do; Don't immediately teleport to Progressive Island (home of Yearly Kos convention in this virtual environment) without reading all the helpful tutorials and walkthroughs available on the place that you begin, Help Island, and be unable to find your way back because you forgot the name of the starting island. Not recommended. Oh, and you cannot fly over the ocean, as much fun as that would be. Some invisible wall stops you at the edge of the land. You can sink into the ocean, however, with no apparent loss of life.
Another thing not to do right after fine-tuning your avatar's appearance--Don't search for 'Ubuntu' and then teleport to that user group, getting a free Ubuntu t-shirt and free laptop. Just don't. Stay away from here there. There are lots of really creepy and scary places in this environment (no joke) and they are thusly marked with the rating of mature. Stay well clear of those, never know what kind of weirdness they are up to.
You can build stuff, fly around, get land, listen to tunes, chat or IM with people (IM'ing is private), play games, and on and on. It's like the computer game The Sims, except much slower and lower resolution, or World of Warcraft, without the violence. No real point to going in to this environment; kind of like arguing about who is the better candidate, or why you should use open source to folks that are not particularly interested--in other words, you should love it.