As soon as you have booked your cruise, the very next thing you need to do is register online.  Go to your cruise line's website and provide the information they need to satisfy the requirements various government agencies.  Each cruise line has their own version of a page on their website for collecting this information (see below) but they are all looking for the same stuff. 

The US Deparment of Homeland Security, US Coast Guard and US Customs all want information about you before you get on the ship.  Not long ago that information earned you a place in what amounted to an express check-in lane at the pier.  You had VIP embarkation treatment by supplying this data in advance.   Now, its not an option and you can be denied boarding without a refund if you do not provide this information in a timely manner. 

There are a couple of other reasons you want them to have this information as soon as possible that can benefit you personally too.   If you have booked a guaranteed category cabin as opposed to an assigned cabin, you stand a better chance of receiving a complementary upgrade the farther in advance you register online.  Also, you'll have the opportunity to look at how the cruise line has your name spelled and your birthdate entered in plenty of time to make any changes if necessary. 

Guests often confuse this online registration for the authorities with the registration information they supplied to their travel agent at the time of booking.  These are two different registrations, both of which are required to complete your booking.