If you do a Form 4 (commercial can), the store will help you with the paperwork because it's in their interests that you get your can.
If you're doing a homemade can (Form 1), then register on the ATF page for an account and you then file online using that. Here's the registration-page - the Register button is at the bottom:
https://eforms.atf.gov/EForms/faces/use ... login.jspx
If you're going to build your own, you CAN buy the components beforehand - you just cannot drill any holes until you're approved. 3 things about your can you need to determine:
1. What caliber hole you're going to drill eg. .30-cal
2. What length the can will be once assembled (you can submit for a longer can and end up making a shorter one, but not the reverse)
3. How you're going to mount it to your firearm(s)