It would largely depend on the wireless router you're using (DLink? LinkSys? Apple Airport Extreme? Some other brand?), but it largely comes down to setting a password and encryption on the router itself.
Read through your router manual(s) about encryption and passwords, looking mainly for "WPA" or "WPA2" password-level encryption. If you can set that up with a good, long, obfuscated password, you're pretty much protected!