You will need a set top converter box.. at one time you could get a coupon from the government for a free one (or maybe they were $25 or something), no longer.
I put antennas in attics all the time without issues. Look at antennaweb and see what antennas they recommend for your area. In general what they recommend should work though I have found that antennas all pick up different channels in different ways and I usually do a site survey first with a meter to determine what your starting signal strength is for each channel then find the appropriate antenna to get the desired signal. Denver still has a couple channels on VHF so a UHF/VHF antenna will be needed. Eventually the FCC will do away with all VHF channels and everything will be UHF only.
Castle Rock is a ways out. I would start with a medium directional antenna.. Don't get sucked into the indoor amplified Terk antennas and others like them, they suck unless you live within 10 miles of the transmitter. Are you down in the valley? Can you see Lookout Mountain? If you are down in the valley it is possible that you could be in a shadow and not be able to get a signal.
Oops.. just saw you were Lakewood and someone else was Castle Rock.. Green Mtn and the Golden Mesas can still cause shadows in Lakewood though. A small directional might work well in Lakewood.
I would also wait a few weeks until the leaves on the trees are fully out. They will knock down the signal over far distances.