How do I use the ethernet cord to connect to the internet?

I know how to plug it in so dont be a dumbass and tell me to do it, when I plug it in the computer recognises it and says that the source is connected. however I am already connected to a wireless source and it wont let me select the ethernet source. when I click it it doesnt do anything and if I turn airport off and have no wireless connection it still doesnt hook up.