You will need to know what the SSID(the name of the router) is and any security passwords that are assigned to the router. If you do not know what the SSID is you can connect to and look at the router settings by plugging into the router with a network cable and using Internet Explorer or fire fox type 192.168.1.1 in your address bar. Go to the wireless settings and you should be able to see what the SSID is, this is also where you set up your security for your network which I highly recommend. Use WPA encryption and use a key generator
(select maximum security you can copy and paste the key into your router settings)to generate a difficult key. For ease of setup you can copy the key you generated to a txt file using word pad or Microsoft word so you can transfer the key to a laptop. They keys generated are very long and some use characters not readily available on the keyboard. In the off chance this is not your router and your trying to "share" internet its illegal.
Systems and Network Engineer
Certified CompTia Network +, A +