Doma means at home. Doma is always an adverb. U tebe doma. - At your home. U něj doma. - At his home. Home is domov. Domov is a noun. Tohle je můj domov. - This is my home. Kde je to tvé doma? It is incorrect. Is it supposted to mean a question "Where dou you live"? If you want to ask somebody for a place where he/she lives just say: Where is your home? Or: Where do you live? I hope it will help you. :-)