Self Assigned IP address - stopping connection

I can connect both my Mac and MacBook Air to my network (home) via wireless however if I try to turn off the WiFi and connect via ethernet I am told it can't connect because it has a "self assigned IP address and will not be able to connect to the internet" My Configure IPv4is 'using DHCP'

I'm not a network guru so any help would be appreciated - in basic terms lol