In order to connect your FlashRouter to the Internet in a single router setup you must have a modem from your ISP with an active Internet connection.
Connect an Ethernet cable from the open port on your modem to the WAN/Internet port on the FlashRouter.
Make sure that you do not connect your modem to a LAN/numbered port on the FlashRouter.
Once you have properly wired your modem and FlashRouter you should reboot each device. After they finish powering up you should have an Internet connection through any FlashRouter wireless network, or through a wired connection to any LAN/numbered port.
Note for PPPoE connection users:
For most users the Internet connection will automatically pass through to the FlashRouter by simply connecting into the Internet port, however in some cases a login is required for Internet, as is the case with PPPoE connections.
In Tomato you can enter PPPoE information by accessing your router and going to the Basic > Network page. You can then change your WAN/Internet Type to PPPoE instead of DHCP and fill in your PPPoE login information.
In DD-WRT you can navigate to the Setup > Basic tab and change the WAN Connection Type from Automatic DHCP to PPPoE and then fill in your login info.
If you are unable to establish a PPPoE connection in this manner you may need to contact your ISP, as FlashRouters can not replicate the setup to troubleshoot ISP specific issues. The simplest solution when this happens is to keep your current router in place to establish the PPPoE login to your ISP and you can then follow the Dual Router Setup instead.
Additional helpful links: