Creating custom user registration form sometimes can be a little tricky, but lets try to create a custom registration form in Drupal 7 with just a few steps.

First we add a few lines of codes to template.php as follows

function themename_theme($existing, $type, $theme, $path){
    'render element'=>'form',
    'template' =>'templates/user-register',
return $hooks;

function themename_preprocess_user_register(&$variables) {
  $variables['form'] = drupal_build_form('user_register_form', user_register_form(array()));

Then creating the user-register.tpl.php and place it inside templates folder in your theme, write the following in your user-register.tpl.php:

  print render($form['form_id']);
  print render($form['form_build_id']);
  print render($form['account']['name']);
  print render($form['account']['mail']);
  print render($form['field_yourcustomfield']); // Your Drupal 7 profile field if any.
  print drupal_render($form['actions']); 

Clear your Drupal cache and visit your user registration form to see the changes.