Click this link to access Cloud Party. Choose a username (usernames are not unique in Cloud Party and they are publicly viewable), pick an avatar and click "Enter without account" to log in as a guest. After loading you will appear in Starter Island. You will be able to chat to users, browse other islands, experiment with your outfit and complete the basic tutorials.
To add other users as friends, buy items on the market, save customized avatar looks, and purchase land and use the advanced build mode, which allows you to save your builds, you need to create an account. Click the "Create Account" on the top right; you can sign up with an email address, or by using your Facebook or Google + account. If you sign up with an email address, you will still need to verify your account to access building features. This can be done by giving credit card information (for verification only) or by using Facebook or Google +.