How to debug: How Can I see the form arrays with its components (fields) in a loaded page? I would like to changing a title of a button in a form, and I would like to have a tool for debug, like devel or another, so I can see in real time, after a page is loaded, what is the structure in forms, subcomponents, name of every field.

So I know how could add a line to the code in my custom module to change the title of a form button.

How would you do it? Steps, method, tools, please share.

Note: These are my first steps in programming modules, changing simple things for now.

Thanks for your suggestions!
Debugging in Drupal can be a bit frustrating when you first start out but you'll get the hang of it quickly :)  

Debug is a really convenient Drupal function to output your form array (or any array/object) from any custom module/template. 

In the case of your form, you can invoke hook_form_alter in you custom module to call the debug of your form. (You can find the form ID by looking at the page code and replacing the - with _. For example "user-login-form" would be "user_login_form" here)

 function YOUR_MODULE_NAME_form_alter(&amp;$form, $form_state, $form_id) {
   if($form_id == 'your_form_id') {
     debug($form,'form',TRUE);   
   }
 }


Then (after clearing caches) once you visit the page with the form, Drupal will output the $form array as an entry in your watchdog log (admin/reports/dblog).