In order to edit pages, you will need to confirm your email address. A confirmation email will be sent to you when you register. You may need to log back out and log in again for the confirmation to take effect.
If you didn't get a confirmation email, or need a new one, log in, go to Special:Preferences, scroll down, and request a confirmation email.
Note that not all pages are editable - protected pages say "View Source" instead of "Edit".