To become a helper you must have 20 points on the forums ACTIVE on forums AND VIP! Than you go up to Application and than after clicking on that you will see a thing saying apply for helper
As Bethany said, if you meet the requirements stated in that guide thread, you can go to the 'applications' tab and click 'apply for helper' and then just fill out the application form. Frequently people difficulty applying because they don't have their VIP rank on the forums. To get your rank to display on the forums, do /link <email you used for the forums> and boom. (It may take a while to update).
If you have any other issues comment below on the thread.