Would Like To Use my Time Capsule as my Main Router Instead of my NVG589
I'm going to take it that you're reading something along the lines of these instructions from dslreports.com. If so, to answer your questions:

1) What you're looking for is if the public (WAN) side of the network is using their own DHCP-allocated address pool for the network branch your area is provisioned with. This would be IP addresses that start with 10.x, 192.x or 172.x. These are the same IP ranges typically used for LAN addresses issued by DHCP servers on internal networks. The instruction is to use one of those address ranges that does not match that of the WAN side of the AT&amp;T modem. So let's say that your modem has a 192.x.x.x WAN address: set your Time Capsule to use the 10.x.x.x address range in its DHCP settings. This will prevent matching and conflicting IP addresses as packets are transferred through the modem, as the modem is performing a basic NAT function in relaying the packets.

2) Yes, you need to disable any of the port forwarding done on the NVG589 since all the packets will now need to be handled by the Time Capsule. @colomb's answer shows the configuration section of the Airport Utility where you would need to set up the new port forwarding rules, as well as what mode to operate the Time Capsule in. The "Network Options" button at the bottom of his screenshot is where you will set the DHCP address range for the Time Capsule as well as enabling the NAT Port Mapping Protocol.

The end result of all of this is that the NVG589 will now effectively be just a modem for the AT&amp;T service and your Time Capsule will be your router and DHCP server with your port forwarding configuration.