The first step is to find a TV channel that isn't being used in your general vicinity. Although after the 2009 DTV transition in June a lot of "whitespace"(i.e. unused television bandwidth) became available, most of this whitespace is still legally dubious and many of these channels are still tied to the original owners via legal identity and copyright. However, because of this legal ambiguity a lot of free space is still up for the taking.
The best option for finding free space is the FCC's own search engine for these things:http://fjallfoss.fcc.gov/prod/cdbs/pubacc/prod/sta_sear.htm
Simply type in the channel (try a range of 14 to 35) and your state, along with the type of broadcasts available. Try to avoid channels that correspond to channels in your area (i.e. CBS 2 in New York). While they might be free because of the DTV transition and broadcasting in a higher channel range, they are still virtually broadcasting through that channel and might also have dibs on it for visual identity's sake.