Have you got the literature which came with the router ? Or download the full manual from the maker.
You need the IP of the router (often 192.168.0.1 or check by using IPCONFIG /ALL in a DOS box from Command Prompt). Put the IP into the address bar of your browser and you should then see a login box -- the user name is always admin. The password may be left blank or be something like password or admin -- see router literature.
Once in go to wireless settings. You need to experiment with channels because some will work better in your environment than others (regardless of interference issues).
To deal with interference you need to select a channel at least 5 channels away from the strongest neighbouring signal.
Obviously the two different issues -- strength of signal due to environment versus interference from neighbours may involve some troubleshooting.
To give a practical example, I live in an urban area where I can detect around 20 wireless networks -- though most not very strong.
My router is about 40 feet away from the computer and down one floor and through at least three walls. Channel 1 is weak. The shop below the flat has a strong wireless signal on Channel 11 -- so the only choice left to me is channel 6. However, my friends are closer than me to someone else who has ecently moved in next door and is using Channel 6. Well, right now, as I am furthest away from the router, we've agreed to stay on Channel 6 for the time being.