Your problem is the the modem is actually a modem/router. The easiest setup is to go from one of the LAN ports of the modem to an LAN port on the router. Then you have to decide which will be the DHCP server, and turn DHCP off in the other. So for example, if you want the modem to do the DHCP but not be the wireless radio: turn off the wireless radio in the modem, turn off the DHCP in the router, assign the router a static address in the same network range as the modem -- you enter that in both the router and modem. (so if the modem is the gateway at 192.168.0.1, make the router 192.168.0.2, and allow the modem DHCP to assign 192.168.0.3 to .254). Set up a security type (WPA2 personal with AES encryption) and password in the router. Write the password down, all wireless connections will need it.
Now the router will act as a wireless AP and switch (its other ports will be on the network also).
edit -- and if you have any other issues please post the model of the modem and the router.