How to install the Second Life software
Second Life and Teen Second Life use the same client software - so these instructions apply to both Second Life and Teen Second Life.
Check that your computer can run the client software
To find out if your computer will be able to run the client software, check the System Requirements page on the Second Life website.
Download the client software
Go to the Second Life download page and select the appropriate Second Life Installer. These tend to be big files (30Mb for the Windows installer) - so allow plenty of time if you are using a slow internet connection!
Install the client software
Once you have downloaded the Second Life client software, double-click on it to open it and then follow the on-screen instructions. You will need administrator privileges on your computer to install it so, if you're not the computer's owner, you need to find the person with the computer's password.
If you find that you have firewall problems then check out the Second Life firewall configuration info page
For the seriously techy (you have been warned) - Second Life needs to connect to ports 443/TCP, 12043/TCP, 12035/UDP, 12036/UDP, and 13000-13050/UDP. You should configure your firewall to allow outbound traffic on those ports, and related inbound traffic.
Still stuck ...
If you get stuck on any part of this process then post a request for help in the Second Life help discussion. Remember to check to see if someone else has already asked the same question - if they haven't then give as much info about the problem as possible in your message.