Usually your ISP (Internet Service Provider, the company that provides you with access to the internet) provides you with one or more email addresses which can be used to sign up. If you don't pay the bills for your internet connection, please ask the person who does.
If your ISP doesn't provide you with an email address, you can always borrow one from a friend or relative who doesn't play. Once you have logged in for the first time, you can enter a free contact email address which will be used if we need to contact you.
Besides ISP addresses, work or school addresses and admin@ or webmaster@ addresses from privately owned domains are also acceptable.