How to let user add multiple sub-content fields in the main content? If I want user to create a main content which can be able to add multiple sub-content for example
I wanna do apartment renting website. User have to create the apartment details which contain these fields; Title, Description, Address, Facilities etc.
As 1 apartment usually have multiple different units, user have to add their available units in the apartment node which contain fields; Unit title, room size, bedroom, bathroom, price. And they can add many units as they want.
The question is
 what module should I use? Can I use Organic Group module or something simpler then that?

I'm new for Drupal, learn by myself from scratch, anyone please point me the direction.
If apartment is one content type, and unit is another content type, you can use entityreference module to reference units in apartments. You will create in apartment content type - entityreference field with unlimited values, type of referenced entity would be unit content type - user will be able then to add as many units to his apartment as he wants.

You can even create a view of entity reference display, and use it for selection of available options in your entityreference field.

http://www.lullabot.com/blog/article/entityreference-multiselectors - also something about multiselector widgets for entityreference field.