Most milcomers will tell you to learn to use the search ranges in your scanner; but that tends to quickly frustrate newcomers (since a lot of traffic is of short duration and there's lots of dead air...), and to me, at least, it isn't good advice for folks just starting off. So I have a few suggestions that will hopefully produce results;
a. First off is the equipment necessary. Thankfully you don't need to break the bank to get into this segment of the hobby. We have a Wiki article on the subject that will get you thinking in that direction...
http://wiki.radioreference.com/index.php/VHF/UHF_Military_Monitoring
b. Now for resources - on that article is a link to the various ARTCC centers in the US. Pick the one closest to you and concentrate on only the UHF frequencies. Ignore any of the FAA ATA100 listings; they tend to be unreliable and out of date.
c. The DoD website remains probably the best source of print (OK, electronic print) material, in spite of certain sections being dropped; note that new editions are due in about 2 weeks;
http://164.214.2.62/products/usfif/enroute.cfm
at a minimum, get the AP1B planning doc, IFR, VFR and Caribbean supplements and perhaps the FIH as well (all at the bottom of this page)
http://164.214.2.62/products/usfif/charts.cfm
Get the AP1B Military Training routes charts (again at the bottom of the page)
d. I have no doubt that dedicated mailing lists on the subject are one of the best ways to get your feet wet without making the real investment. It's a good way to scope out activity before you make your decision...
http://groups.yahoo.com/group/AirCommSouthwest/
-and-
http://mailman.qth.net/mailman/listinfo/milcom
Posting a question to this well-known list will no doubt bring results, if no one here responds...I would also check into...
http://groups.yahoo.com/group/SEARM-GROUP/
http://groups.yahoo.com/group/ArizonaScanner/
and there's Grove's frequency directory as well, although it hasn't been updated lately.
That should get you started...73s and GL..Mike