You simply click here to reach the home page. When you are here you can answer a question straight away, but to sign up you should go to the box a little way down on the right of the page (It says 'New To Qsponge? Sign up!') Then just put in your name and your email address and just follow the instructions after that. You also have to agree to adhere to the site's rules, eg not posting anything offensive.
Once you have joined you may wish to put some information on your profile - a little about yourself and your interests, and perhaps a photo. When you start asking and answering questions, on your personal page you will see a record that shows how many questions you've done. Your QSponge level goes up depending on how many questions you answer.
You can answer questions at random, or click on any special category that interests you and see if there are questions in there that you can help with - and you can ask about anything you like!