How to call a drupal custom module function from a basic page?
I guess you want to create a menu callback which will call your form right? 
Here is how to-

Create your menu hook like bellow:

function mymodule_menu($node) {
  $items['my-frm'] = array(
    'type' =&gt; MENU_CALLBACK,
    'page callback' =&gt; 'drupal_get_form',
    'page arguments' =&gt; array('my_form'),
    'access callback' =&gt; TRUE,
  );
  return $items;
}


Than create your form like below:

function my_form() {
  $form['submit'] = array(
    '#type' =&gt; 'submit',
    '#value' =&gt; t('Submit'),
  ); 
  return $form;
}


And after that here is your form submit:

function my_form_submit() {
  drupal_set_message("Hello World my form submit hook is called here");  
}


Do a clear cache and navigate to yoursite/my-frm link you should see your form.