I'll be comprehensive for OP's sake too.
There's usually regular registration and pre-registration. Pre-reg is done online, and often discounted, but there's a cutoff date for when your con stops taking pre-registration. Your attendees are emailed a "receipt" of sorts, which they must present along with ID at the con in order to pick up their pre-reg badge.
Usually, registration will be split into two sections: pre-reg and normal reg. Pre-reg, like I said, is all "show your receipt and ID, get your badge", and normal reg is where attendees bring cash (or card, if you have something like those Smartphone/tablet things to accept card payments) and pay the normal at-the-door price.
It's up to you whether you also want to get info from people buying at-the-door registration badges. Personally I don't see the point and feel like it slows the line down, which is something you want to avoid. Make sure you have a section off to the side with Sharpies for people to write their name/nickname on their badge.
On a related note: Make sure your attendees are aware that they and only they are responsible for their badge. Your con should have a lost&found where people can turn in lost badges (and other discoveries, of course). Usually this is located in Con Ops (the room where most of the staff operate when not out on the floor doing other business, and where the non-security help staff works). Data is recorded with each turn-in: What is the item? Where and when was it found?
When someone stops by L&F to retrieve their badge, they must say the name written on the badge. If the badge is completely lost (no one turned in a badge with that name), or the attendee didn't bother to write anything on their badge, it's not the con's responsibility and they'll likely have to buy a new badge. Make sure this is outlined in the con's program booklet that completely lost badge = buy a new badge, and having a sign about this near reg also helps.