To get involved in iSpot you'll need to register on the site. Registering is free and only requires a user name, email address and password. Watch the video on how to register, then. (Right-click on the link to open it in a new window or tab.)
The next thing to do is to get outside and make some observations. You need to note down what you've seen, where you've seen it, and when you saw it. If you can take a digital photograph of it that is very helpful, and allows others to share what you've seen.
For help and advice on what to spot, where to see it, and how to identify it, go to the top of the page and click on the groups menu and select which of the eight iSpot wildlife groups you are interested in.