As BA says, we'll need more info.
But just a few basics in no particular order;
1 Hide the SSID and change it.
2 Use the highest security you can, WPA/WPA2, change passwords and made them complicated. If your access point can only do WEP, throw it away and get another one. (Although check if a firmware update might help first)
3 Now take a moment to review and re-assess that it's not something of your own that causing the queries. IP Camera, TV box - many things need dns. It's could literally be a case of disconnecting every item to test.
4. Enable Wifi isolation and any other measures your WAP might have.