If you’re talking about needing a person, place or thing (ie: a noun), always place a de after precisar. The de is there because you are literally saying, I need of something.
Eu preciso de você! (I need you!)
Ele precisa de dinheiro. (He needs money.)
Você precisa do meu telefone? (Do you need my phone?)
If followed by a verb you don’t need the de. But the following verb will always be in the infinitive. You are saying, I need TO do, TO say, TO eat, TO play ETC.
Eu preciso correr. (I need to run.)
Ela precisa estudar. (She needs to study.)
Você precisa descansar? (You need to rest.)