The PIX 501 will come, by default, with a DHCP client enabled on the outside interface, a DHCP server enabled on the inside interface, and NAT configured using the outside DHCP address. You shouldn't need to do anything other than plug it in to the Internet connection, and plug your PC's into the back of it.
Having said that, the commands to do NAT on the PIX are the NAT and GLOBAL commands. The NAT command defines the traffic to be NAT'd, and the GLOBAL command defines the address to NAT it to.
For example,
nat (inside) 1 10.0.0.0 255.0.0.0
global (outside) 1 interface
will NAT any inside traffic from the 10.0.0.0/8 network to the IP address on the outside interface. The command reference is here:
http://www.cisco.com/univercd/cc/td/doc/product/iaabu/pix/pix_60/config/commands.htm
