Well if your the owner of the this box, ie take a "modem/router/gateway" if this provides dhcp to your current network and you want put pfsense and your network behind this "box" then you can just let pfsense use dhcp on its wan (which is the default) or you could set it to be static depending on the network currently behind your "box"
That would be up to you.
Normally pfsense would replace this "box" and get dhcp from your ISP via a modem or a gateway in "bridge mode" so that pfsense gets a public IP on its wan from your ISP. But it does not have to be on the public internet it can be behind your box doing nat.
Then in general you would place the rest of your network behind pfsense. Where you could use the default network on pfsense lan of 192.168.1/24 or set this to be whatever you want. The only thing to make sure of is that network on the lan does not overlap the network on the wan of pfsense.