Hello, I would like my customers to be able to add fields to a form, with differnt values, etc..
Basicly, i would like them to be able to add any type of fields and customize their forms; like google forms for instance. And of course, i need the databse to be able to insert/update etc

Do you guys have any example?

Not too hard to do, but you buy a few cheap ones on the net for about $5.

Google form builders. That should give you the front end. You'd have to write the backend.

Failing that Wordpress has a few form builder plugins such as gravity forms...