how to add input field into admin post-new

Hi there, i’m making my own theme and updating a little bit the admin panel.
I want to do a little update to the post-new section in the admin panel.
I would like to know id there is some way to add, including some code (like add_action(…..)) into function.php, an input (or more than one) field so allow admin complete it to add extra information (like a gallery to the post or something else, i dont like to use plug-ins at all).