The night before you arrive in GC your navigator will list the time/place that tendering tickets are available in the morning. If you have rented a car and want to be one of the first off the ship I would suggest arriving approximately 15 minutes earlier than the time stated. Then you should be assured of getting on the first tender and able to start your day and make the most of it.
They give you your ticket, line you up in order of your tickets, when it is time, you follow in line together to the tender area, you get in the little boat and ride over to GC dock. All very easy. Quick and painless.
After a certain time in the morning tender tickets are no longer needed, they just use them first thing so there is no confusion about who got where first and everyone gets off the ship in an orderly fashion.
GC is lots of fun. Enjoy!