Very basically you need to be able to log into your router - so do you know how to do that?
I open up a browser window and type in
http://192.168.1.254 or I used to use
http://speedtouch.lanObviously this depends on your router and you need to check the docs that came with it.
Do you have a username and password? If you have never logged into your router then it may have a generic password or no password at all - this is not good - so you need to login to your router anyway to make sure you set up a new password. Usually you log in with the username "Administrator".
When you manage to log in then have a look at the menu options to do with your wifi connection (mine is "Home Network") - you need to find your way to the "Interface" and click on the
configure option and change it from automatic to manual and then select a channel (usually 1,6 or 11) - of course if you have any PC's attached by wifi then you may have to reconfigure these to connect to the new channel.
Ideally you should make sure that you do not allow new devices to connect automatically (although this is the easiest option at first to make sure everything is working ok) but at some point I would advise that you only allow connections to your router by the MAC address of the relevant device (this is more complicated but maybe something you could look at later).
You should also make sure that you do not broadcast your network name (SSID) and certainly do not call you network by any name that identifies it as belonging to you.
Whilst you are logged into your router just have a check at what encryption you are using - if it only supports WEP then get a new router ASAP that supports WPA.
And make sure you set up a new password on your router!