(Note: The FastPass entry is right below this one, but you have to scroll to see it.)
To be fair to park guests, Disney limits you to one FastPass until your window starts, or 2 hours from the time the FastPass was issued, whichever comes first. You can then get a FastPass for another ride, but you cannot hoard FastPasses all at once for every ride you want. Therefore, as you enter the park in the morning, head for the ride you want most, and get that as you first FastPass. Or ride it right away, if the line is not too long yet.
FastPasses are available for select (but not all) attractions in the parks.
The FastPass machines will only accept park tickets that have been scanned at the park entrance. Therefore, you cannot take your entire family's batch of tickets with you to the park and get them FastPasses while they sleep in at the hotel. This is another way Disney ensures a fair distribution of these precious commodities.