Adopting a U.S. Soldier

They have email access, so you could do it that way once you get their email address. The first couple deployments I did, we didn't have internet/email access. We could pay something outrageous like $50 or so for a 10 minute phone call with crappy reception and about a 3 second delay. It really sucked then. The last few I've done weren't so bad. The very last one I did in Iraq, we got lots of care packages from people. At Christmas, some people sent us all stuffed stockings.
As Goldfish said, it's not always a penpal type thing. If you send mail through a "Support the Troops" program, they may not have your address, or they get such a large volume of letters that they can't reply to all of them. Just know that they are all VERY much appreciated, whether you hear back from them or not.