Which one is correct when I need something
Should be say I need or I want
If you are talking about something, that's is really nesessary for you (for your life, without which you just can not live) like water, bread, milk, house, bed etc - it means that YOU NEED them.
If you are talking about something that can satisfy your wish, but it's not so necessary for your life like IPHONE 6, Lamborgini, new TV - it means that YOU WANT to have them.
I want a car, but don't need it.
I need food to live, but sometimes don't want to eat.
we usually use word "want" to express inner desire , but use word "need" to express some certain demands.
Thanks all for your information
"I want a new suit. But I dont have money."
---> A want is something you would like to have. It is not really necessary like food and air to live.
"I need a suit for job interview"
---> A need is something you have to have. As above example, a suit is very necessary. Your interview will be completed without that suit.