Not sure what you mean by satellite. You don't need the 3G plan to use GPS, though it may improve the time it takes to get a location lock.
For data services on 3G:
Go into Settings >> Cellular Data and turn it on. Now you'll see a View Account, or maybe Add Account. Not sure. Just tap on it and follow the instructions. You will need your Apple ID, and it will charge the credit card associated with that ID. At least I'm pretty sure that's how it worked. Been a while since I activated mine.
You have the option of two plans.
$14.99 gets you 250MB for 30 days.
$29.99 is 2GB for 30 days.
These plans auto renew, but you can stop them or change them at any time right on the iPad. If you stop your data plan service will actually keep running until the end of that 30 day cycle. You will not get any money back for closing it early, but you won't lose any use time either. If you start a new plan it begins immediately, there is no rollover of data from the old plan. So you can lose some data that way if you are not careful.
Say you had a 2GB plan going and decided to spend less. When you activate the 250MB none of the 2GB is going to roll over. It is gone. That's a bummer, but in compensation, you will never be charged for accidental over usage. When/if you use all your data allowance the 3G service just stops working until you either add data with a new plan, or it auto-renews at the end of the month.
Note: I might have some minor details wrong. It's been a while since I reviewed this info and my memory has been know to slip, just a little, every once in while, no more than twice a minute.