How to use the html <form> tag with JavaScript?

Question

I've started to learn html and css a few months ago and now I'm learning JavaScript so I can use it on my websites. So, the <form> tag has the id, name and type, but witch one is the variable? And how can I use that variable on JavaScript?
How can I write something on the screen with JavaScript with my css styling?
Thanks to everyone how answers and sorry if there's any grammar error above as English is not my mother language.

Answer 1

The correct use of form tag is to send data to the server, with HTTPRequest method of POST.
If you want to process it with JavaScript, you are trying to handle the user input at client side.