The default configuration has lan as a DHCP server. You can simply create a wifi AP and attach it to LAN to have wireless devices on the LAN.
Install USB Ethernet drivers matching the chip in your adapter. After the driver recognizes the hardware, an "eth1" port should appear in the list of physical ports (go to the lan interface, click physical tab, pull down the box). If you add eth1 to the lan it will work like a lan port. This is a good way to test that your drivers are working. Then remove it from lan.
Now having an extra Ethernet port, add a wan network. Name it exactly 'wan' (not 'WAN'). Protocol is DHCP client. Physical device is eth1. Plug that into your modem or other existing network. It should now be possible to route to the Internet from lan.