To create a free account, go to http://secondlife.com/ and click on "Join now". A page will open offering a choice of default avatars. Choose one, and the next screen will prompt you to pick a user ID. The next screen asks for your birthdate, email address, and the password you want to use to log into Second Life. Click "Create Account".
Second Life runs in a viewer. The official version can be downloaded here: http://secondlife.com/support/downloads/?lang=en-US
There are a number of third party viewers as well. If one viewer doesn't run well on your system, try another.
Keep in mind that when you log in, your last name is Resident. So for example, if your user ID is Barney77777, you would sign in as Barney77777 Resident. Once you log in for the first time, there are some tutorials showing how to move your avatar, chat with other avatars, etc. In some viewers you can add a display name which can be almost anything. You can also change your avatar into just about anything.