Several mid-to-upper price units are "Frequency Selectable". That is, they allow you to change the channel, usually with little switches on the transmitter and receiver, to one that doesn't interfere with anybody (or get interfered with). The rest of these give you a choice of frequencies for you to operate on. Once you buy it, that's the channel you stay on.
The lower-end ones don't give you any choice at all. But, you could be lucky. Actually, they try pick frequencies that are not heavily used but with the development of so many new uses for wireless technology, open frequencies are becoming VERY scarce.
You might talk to a local theater (legitimate, of course) or a concert venue (maybe not-so-legitimate) and ask the sound man which channel(s) are good for the area you're going to perform in. There are two major divisions, VHF (VERY High Frequency) and UHF (ULTRA-High Frequency) and several bands within these, each having several channels apiece. So, you need to decide which of the two groups you're going to go with (this may be decided by monetary considerations) and then you can do more research on the specific channel(s) you need.