Question

What wireless channel should I use?

Answer 1

There's a program called inSSIDer which you can use to evaluate which wireless channels would be best suited for a network. You want to look for channels which aren't in heavy use.

Answer 2

When looking at wireless channels, there are a couple of variables to keep in mind:
The number of other networks using what channels
The strength of the other networks on other channels

So generally speaking, channels 1, 6, and 11 are used by default. The reason for this is because there isn't any adjacent channel interference when you have all of your networks spread out in that manner. If for example, a network was using channel 3, that network would interfere with the signals one channel 1 and 6, and the bandwidth of those channels would have to be shared amongst all of those networks. This isn't optimal. Ideally you should look for a channel with less hosts who have weak signals. I know that in my area, channel 11 is the best channel to be on, because the hosts using that channel have weak signal strengths, which results in a lesser chance of those signals interfering with mine.