How do I calm my dog down?

My dog--a 5 month old German shepherd mix of some sort--is wreaking havoc in my living room. He is running around and around in circles without stopping. He won't sit down or anything, so finally, I tackled him. He then kept biting/nipping me and he barely calmed down. We cannot take him to classes because we can't afford it and because we need to calm him down RIGHT NOW. Please help!