I am searching how to press the Subscribe button from Youtube Subscribe Form, but I didn\'t find anything that can help me... I am new in JS, so I hope that you will help me.
This is impossible. The Javascript safety policies do not allow crossdomain access to the document of an embedded (i)frame.
You won't be able to manipulate the content of an external i-frame. It is called the Same Orgin Policy. You are going to need to ask the user to click on the button.
That would be a really bad practice to click on button without user's consent.
--EDIT--
If you want to subscribe the user to your channel without the user clicking on the button, read the documentation about the Youtube API on how to subscribe a user to a channel. That's your best bet.
Hope that helps.