In the US, a Wireless G router uses part of the 2.4 GigaHerz frequency band, and there are 11 channels available, numbered from 1 through 11. If you have not told your router what country you are in during setup, you may not see any choices other than channel 11.
If you see only one competing network, on channel 11 for example, the best performance for you will be to go to the other end of the band, channel 1. If there are many different networks in your range, spread over a lot of channels, look for a channel which is not used and does not have a strong signal competing on either the channel just above or the channel just below.
The other thing to watch out for is that the 2.4 GHz cordless phones (not cell phones) use the same band, and may allow you to select a channel for the phone or may use a "spread spectrum" or "frequency hopping" process to try to avoid interference. If you get phone interference and you can set the phone channel, try to avoid that channel or to change the phone channel. If you cannot change the phone channel, try setting your router to channel 1 or channel 11 to avoid the middle of the phone band.