This article will explain how to use your TP-Link N router (as you have not mentioned any in your question) as an access point. The main router will be connected to the TP-Link N router via LAN port (as seen below). The WAN port is not used for this configuration.
Main Router Access Point
Connect your computer to a second LAN port on your TP-Link N router using an Ethernet cable.* Login to the TP-Link web interface through the IP address listed on the label on the bottom of your TP-Link N router
Note: Though possible, it is not recommended to attempt this process over Wi-Fi
Go to Network>LAN on the side menu and change the LAN IP address of your TP-Link N router to an IP address on the same segment of the main router. This IP address should be outside the main router’s DHCP range.
Example: if the DHCP of your main router is 192.168.2.100 – 192.168.2.199 then you can set the IP of the TP-Link N router to 192.168.2.X (X should be outside your main router's DHCP range), like 192.168.2.11.
Note: After changing the LAN IP address a reboot will be required and you will need to log into the TP-Link N router with the new IP address.
Go to Wireless>Wireless Settings and configure the SSID (Network name)which can be the same or different from the main router’s. Select Save.
Go to Wireless>Wireless Security and configure the wireless security. WPA/WPA2-Personal is recommended as the most secure option. Once configured, click Save.
Note: If using a dual band router, repeat this process for the 5GHz band as well.
Go to DHCP>DHCP Settings and select Disable the DHCP Server. Select Save.
Go to System Tools>Reboot and select Reboot to reboot the device.
Use an Ethernet cable to connect the main router to your TP-Link N router through their LAN ports (any LAN ports may be used). All other LAN ports on your TP-Link N router will now grant devices Internet access. Alternatively, any Wi-Fi device can now access the Internet through your TP-Link N router by using the SSID and Password set up in the above steps.