Wah How to use "Need"

Need is used when you want to say something necessary to do or to have.
I need to do it = I must do it.

Affirmative           
     I need to go now. (It's necessary to go now.)

Negative
     Today is Saturday, I don't need to go to school. 
     or I needn't go to school.

Interrogative
      Does she need to have the description for buying medicine?  
     or Need she have the description for buying medicine? 

The modal use of 'need' is restricted to its present tense-form. To refer to the past you have to use..

Affirmative  (Needed to + v.)  
     I knew you needed someone to look after.

Negative (Did not need to + v)
     He didn't need to go to school yesterday = it wasn't necessary and he knew that and most likely he didn't go.
     He needed not go to school yesterday is incorrect.

Interrogative 
      Did she need to go back to Tokyo. 

Note: When you want to say something that you had done in the past and you think it was unnecessary to do it. You should say...
"Need not have + past participle" or 
     He needn't have gone to school yesterday = he went to school but it was unnecessary.

Lady Antebellum - Need You Now
   "It's a quarter after one, I'm all alone and I need you now.
Said I wouldn't call but I've lost all control and I need you now.
And I don't know how I can do without.
I just need you now..."  

If you want to know the use of need more, click here :

Wah How to use "Need / Want + Gerund (ing)"