So, the first step is to go to the e-registration page of the BIR and scroll down. There you can see a form to be filled up.
What I can say with registering online is that it asks for a lot of information but compared to going physically to the nearest BIR office, getting Tax Identification Number (TIN) Online can save you the trouble of long lines, hot rooms and tiring application. Not to mention that I am facebooking while registering my TIN number. No SWEAT! :)