Hi, Davi. “Need” is usually *not* considered modal, and as such, requires the infinitive with “to”. Also, the negative form and the interrogative form both require the helping verb “do” (or “does” or “did”) in the simple tenses. Most verbs fall into this category, including “want” and “have”: What do you want to do? I don’t want to do anything. I have to go now. Do you have to go? No, I don’t.
Modal auxiliaries, on the other hand, take the base form (infinitive without “to”) and form their interrogative and negative using inversion: Will you be there? No, I won’t. Modals do *not* add S in the 3rd person singular of the present tense. Modals usually have irregular forms, and are defective — lacking an infinitive / base form, gerund, and participles. I will, you will, he will — not “he wills” (“he wills” exists, but has a different meaning - é a sua vontade que.) There is a relatively short list — here are the ones that come to mind: may, might, must, can, could, shall, should, will, would.
Some verbs that are usually *not* considered modals may be used as modals in old-fashioned style (maybe it’s more common in the UK; I don’t know). For example, “need” and “ought”. While you will occasionally see such “semi-modals” used as modals, there’s no reason for you to do so yourself.