After your signup and account activation, you are ready to go. Click on the login link on this website and login with you user credentials.
Now login, it's time to add your first server to monitor. Click on the Manage button in the menu, on your left and click the "add server" button. Fill in all the import details, like: name, hostname and alert to group. Press ENTER or click the save button to add your server. After you added your server do a right click with you mouse on your server and choose services to monitor. Most people would check the http (webserver) and ping service.
Be sure to check your contact information. Feel free to add more contacts or contact groups. Don't forget to add contact any new contact to one or more groups!