You could setup an access point by following these steps :
1-Find the IP addresses of your existing gateway/router and clients : connect a pc to it and use (Start > Run > type: cmd) ipconfig to get its IP (gatway ip).
don't forget to - set your client computer to obtain its IP automatically (default behavior in Windows)
2-Configure the wireless router/AP
First, you need to change its internal/LAN IP address to an unused address in the same range/subnet as all your other LAN devices to avoid any collisions. Second, you need to disable the DHCP server on your new AP, so there is only one DHCP server on the network.
for exemple if the gateway/LAN router ip is set to 192.168.1.1, and it is serving dynamic IPs via DHCP in the range 192.168.1.10 - 192.168.1.100. I have to use any other address in the 192.168.1.X>100 range for the access point.
3-Connect the AP to the LAN
first reboot the router and Use a LAN port on the new wireless router, and connect it with a network cable to one of the LAN ports of the existing gateway. Make sure not to use the "Internet/WAN" port on the wireless access point!
4-conenct your wireless device and test
If both your main gateway and access point have wireless capability, you can use the same SSID, same security, and different non-overlapping wireless channels to extend the range of your wireless network and allow clients to connect to either one automatically.