"What kind of grill do you have?"
Translation:Millainen grilli teillä on?
You have two question words in your sentence, "millainen" and "onko", which isn't necessary.
Also, in statements and questions with questions words such as "millainen", "mikä", "kuka" etc. (i.e. not the question suffix -kO) the order of the subject and predicate verb stays the same.
Sinulla on grilli.
Millainen grilli sinulla on?
Miksi sinulla on grilli?
English: S + V -> V + S? (You are x. - Who are you?)
Finnish: S + V -> S + V? (Sinä olet x. -> Kuka sinä olet?)