If you are talking about an object needed, do not use to. If you are going to say a verb next you may need to. I need a car. I need to drive there. I need to be on time.

you have to say to if you are going to say a verb after 'need' . Don't put 'to' if you're going to say a noun after need. :)

In the sample sentence, "You need to think carefully before you make a decision." The phrase "to think" is a infinitive verbal acting as the direct object in the sentence. Therefore, in your words, putting "to" after "need" in this case is correct.

"you need to" is right