What is the registration number for a drone to fly it outside?

The first thing a new drone owner needs to do is to register it with the government.
Registration costs $5, but you can avoid that fee if you register before Jan. 21, 2016. There is a grace period, but if you're caught flying a drone that's unregistered, civil penalties of up to $27,500 may be assessed (although you'll have until Feb. 19 before they can be assessed).
If you're a U.S. citizen or legal permanent resident and own a drone that weighs between .55 pounds (250 grams) and 55 pounds, you'll have to head on over to the FAA's new webpage and take care of business.
If you happen to have a drone that weighs more than 55 pounds, don't think you're getting off scot-free. Rather, you'll have to use an alternative registration process that you can check out via the FAA's Aircraft Registry page.
Age wise, drone owners younger than 13 will need to have a guardian or representative
register the drone in their place.
Once you register, you'll receive a unique registration number that you'll have to affix
to all of your devices in some fashion or another.
The FAA rules dictate that the registration number must be visible and can be attached via engraving,
a permanent label, or a permanent marker. FAA does note that you can put the number
inside the device's battery compartment provided that special tools aren't needed to open it up.

All registered owners will be issued an FAA certificate (via email) that they must have on their person
anytime their drone is in-flight. The certificate will house a person's name, registration number,
and also the associated dates of issue and expiration.
https://registermyuas.faa.gov/