It depends on the context, because lay is a transitive verb, referring to a direct object, while lie is an intransitive verb, referring to the subject.
Lay means "to put or place something or someone down," e.g. "I need to lay Mark down in his bed, before I drop him."
Lie, on the other hand, means "to relax or recline," e.g. "Mark needs to lie down in bed before he falls asleep, because I can't carry him."