can you use your wireless modem and a airport to extend wifi or do you need two airports?

It is possible, but only if the two will be interconnected by Ethernet. This would be the basis for a roaming network.

If you require a wireless connection, then you would need a second AirPort to accomplish this. The first AirPort would connect to the existing wireless modem via Ethernet. It would be configured as a bridge and also configured to be extended by the second AirPort. You can use any combination of 802.11n AirPort models for either of the AirPort to configure an extended network. Finally, you will want to disable the wireless radio in your wireless modem to prevent Wi-Fi interference between the two wireless networks.