How to set up more than one wireless router?

I have one in my house right now but I have a lot of devices on it right now! So how do you set up more than one wireless router in the house? I wanna set up two more! One is called ZyXel and when I go on my iPhone it connects to it but no Internet and the other is called TP-LINK. How do you connect them both to Internet? My computer is slow so i don't know how to so help please

You don't. You should NEVER use more than one router on any connection. Even if you do, you would need all of them to connect through the one connected to the service, so there's no advantage from that. If you want more wireless connections, and most can handle up to 20 machines comfortably, you add wireless access points connected to the router LAN ports, or a wireless repeater or range extender which is standalone. Most likely any problems from the number of devices is that you are not subscribing to a wide enough bandwidth broadband.

You can use multiple routers provided they are configured correctly.

Unless you have experience in network configuration, it's not a good idea to use additional routers unless you turn off the routing functions and use them as wireless access points. Your wireless signals will interfere with each other if they're operating on the same wireless channel. A router also acts as a DHCP server to assign IP addresses, you only want one DHCP service running.

Dynamic Host Configuration Protocol (DHCP) is an auto configuration protocol used on IP networks. Computers that are connected to IP networks must be configured before they can communicate with other computers on the network. DHCP allows a computer to be configured automatically, eliminating the need for intervention by a network administrator. It also provides a central database for keeping track of computers that have been connected to the network. This prevents two computers from accidentally being configured with the same IP address.http://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol

You'll need to manually configure the second router before placing it into service. The IP address needs changed so it doesn't conflict with the first router, the DHCP functions should also be disabled on the second router.

You'll have to connect a PC or laptop directly to the second router via Ethernet and log into it's interface to change it's IP. You'll want to change this to something that doesn't conflict with the first router but is in the same subnet.

Plug a Ethernet cable from LAN port on the first router into one of the LAN ports on the second router, Don't use the second router's WAN port for anything. Also use non overlapping wireless channels such as 1, 6, 11. You can use the same SSID for everything.

For example if you main router has an IP of 192.168.1.1 and it broadcasts on channel 1.

Set your second router as an access point with an IP of 192.168.1.2 and have it broadcast on channel 6.

Although it is not absolutely necessary, I would also reserve the second router's IP in the first router's configuration or set DHCP to assign a range that doesn't include the second router. For example if your devices had the IPs listed above and you wanted to allocate up to 20 DHCP IP addresses you could set the range from 192.168.1.10 to 192.168.1.30. This way in case the second router were powered off for some reason DHCP would not assign 192.168.1.2 to another device which would cause an IP conflict when the second router was powered back up.