Can anyone tell me when to use zorunda, and when ihtiyaç and when gerek, if I was to say I need something? Which is more used under -say- young professionals? Or is any one of them more street talk or more ´upper class´ or older generation? How do I choose?

Teşekkürler!

Check this thread. There are some explanations related to your question.

http://www.turkishclass.com/forumTitle_6452

Tunci, thank you very much, daha iyi anladım. This was very helpful:

lazım - it is generally necessary

gerek - it is required by someone or something
zorunda - it is required because there is no other choice

But 1 question remains: There is not a difference between PEOPLE who use the different forms? Like old-fashioned people use more ´lazım´ and schoolchildren use more ´gerek´? 

Is there a difference in politeness between the words? Like, if I talk to older people, is it then better to use one of the forms than the other?

I think not, but I like to hear it from a native speaker.

I will add my thoughts from what I have read and some Turkish class notes from a recent course I completed.

Firstly, my understanding is that ´zorunda´ is used with verbs, and the necessity is imposed by external forces or circumstances. It is used by doctors, on instructions for medicines, and is absolutely necessary.

Ali, diyabet bunun için insilün kullanmak zorunda.

Ali must use insulin for his diabetes. 

Ihtiyaç is a noun that means need, necessity, requirement.

Gerek is a noun meaning need, and also has a verb form gerekmek (to be necessary, needed, required)

Bana seni gerek (I need you)

With the verbal forms (Gereklik kipi - Necessitative mood) here are some examples

1) malı/meli

Ben gitmeliyim (I must go, I should go)

Easiest for Turkish learners to remember, used on the streets, often in the verb plus -meliyim, or -meliyiz (I must, we must) form, and considered a bit impolite in the -melisin, -melisiniz form.

2) lazım/gerekiyor

Benim gitmem lazım (I must go) 

Benim gitmem gerekiyor (I must go)

lazım is used in daily street language, and gerekiyor is used by the press in papers and on television, used by academics and in books.

This second category tends to be a stronger form than the meli group, but the strongest necessity is the next group.

3) mecbur/zorunda

Ben gitmeye mecburum (I must go)

Ben gitmek zorundayım (I must go)

Absolutely 100 percent necessary. Zorunda is used more than mecbur.

Mecbur is used for personal, physical, social, psychological, and in theatres to turn phones off. Zorunda is used by doctors and on medical instructions.

I guess if you gave some examples of ´what you needed´ then you might get a more specific answer from a native speaker.

You are wrong here 

You cant say ben gitmem lazım, it should be benim gitmem lazım

or you can say just gitmem lazım without benim.

Thanks for the correction. My mistake 

Thank you KediNero and Henry, that´s exactly what I meant. Examples are

- if my friend asks me if he needs to bring anything when he comes to my house, I would say ´Gerek yok´.

- if I want to ask my friend´s mother if she needs any help making diner I would ask ´Yardıma ihtiyacıma var mı?´

- if I say to my friend we have to leave now or we are late, I would say ´gitmeliyiz´ or ´gitmek zorundayız´, probably the first one because it is shorter.

Right?

i would say like that:

1.) Gerek yok

2.) Yardıma ihtiyacınız var mı?

3.) Hadi gidelim, geç kalacağız.

My thoughts, but wait for a native speaker

1) gerek yok (simple and easy to remember)

2) Nasıl yardım edebilirim? Nasıl yardımcı olabilirim? How can I help you? because if you ask if she needs help, she will most probably say ´no´.

3) Generally I would use ´meli´ unless you were going to work or an appointment, where using ´zorunda´ indicates a stronger necessity.

1) gerek yok (simple and easy to remember)NO NEED or  I DON’T NEED

2) Nasıl yardım edebilirim? Nasıl yardımcı olabilirim? How can I help you? because if you ask if she needs help, she will most probably say ´no´.HOW COULD I HELP YOU?

3) Generally I would use ´meli´ unless you were going to work or an appointment, where using ´zorunda´ indicates a stronger necessity. I MUST HELP YOU and  I NEED TO HELP YOU or I HAVE TO HELP YOU or WHAT CAN I DO FOR YOU or I WOULD HAVE HELPED YOU!