To approve a page, click on the History button.
Anyone with the 'approverevisions' permission will see a link that reads "(approve)" on each row of a page history page. Clicking on that link will set that revision as the approved one.
If you go to the history page, you will see an "approve" link next to every other revision, along with an "unapprove" link for the approved revision; the approved revision's row will also have a star next to it. Clicking "approve" for any other revision will re-set the approval to that revision; while clicking "unapprove" will mean that there will no longer be an approved revision for this page.
Users without 'approverevisions' permission will see a star icon on the approved revision's row.
Normally, every time a user with 'approverevisions' permission makes an edit to a page, that edit, i.e. the latest revision of the page, gets automatically marked as approved. However, when a New page is created, the auto-approval does not happen. The page has to be manually approved the first time to get it into the "approval" system before the automatic approvals are done when a user is in the Administrator or Reviewers group.