Since dhcpd has to hand out IP addresses to clients, it needs to know the range of addresses that it is responsible for. The subnet declaration gives dhcpd that information and more. Assuming you're using 10.0.0/24, the following should get you started and past the error message, but you really need to get into the documentationget into the documentation to go further. Add this to your dhcpd.conf:
subnet 10.0.0.0 netmask 255.255.255.0 { 
   authoritative; 
   range 10.0.0.1 10.0.0.254; 
   default-lease-time 3600; 
   max-lease-time 3600; 
   option subnet-mask 255.255.255.0; 
   option broadcast-address 10.0.0.255; 
   option routers 10.0.0.0; 
   option domain-name-servers 8.8.8.8; 
   option domain-name "example.com"; 
} 
The IP addresses I plugged in above are guesses. You've got to set these properly for your setup.
 
                