Dish network does not carry the subchannels. They only carry the main channel. You should check if the signal is strong enough at your house using TV Fool - Home. If so, put up an antenna.
On DISH channel 20 is 12-3 and channel 12 is 12-1 SD only. If you want the digital broadcasts you will need a digital converter box (with the Gov't coupon) or a DISH receiver with the ATSC tuner built in. A TV antenna to receive the broadcast signal, currently both are UHF but Feb. 17th 2009 KXII is going back to VHF channel 12 so you will need a VHF/UHF antenna in the future.Ch 10 offers 10-1 NBC/10-2 CW network and 10-3 a 24hr weather channel. I guess I should add 12-2 is the My TV network. Depending on where you live Dallas may be receivable which would add about 30 channels of Digital programming.