All 1K can hold intins and some other MP members can also (not sure what that depends on). So unless you're a 1K or one of the few chosen you're pretty out of luck when it comes to holding an itin. there have been threads (and I recommend a search) on how to "cheat" the system by providing incorrect CC info and holding an itin that way. another way is calling the reservation line and they will generally hold a reservation for a couple days - only downside is no online booking bonus and the 5$ ticketing fee chargd once you decide to take the flight. Alternatively you can buy a flight online and cancel it within 24h w/o charge. sort of like a hold feature if you want.

hope that helps and again do a simple search and I am sure all you're questions will be answered in detail as this has been discussed a few times.