You can point an diferent page to login.
Go to: Administration > Plugins > Authentication > Manage authentication
In this page there is an option: "Alternate login
URL". In this option you can make moodle redirect to a custom login page.
Just copy the HTML of your page, make the changes you need, save as an new file and make moodle redirect to it.
If you want to create something new, use this form in the HTML page and you are free to do anything
<form method="post" action="http://yoursite.com/login/index.php">
<fieldset>
<label for="username">Username:</label><input type="text" name="username" id="username" />
<label for="password">Password:</label><input type="password" name="password" id="password" />
</fieldset>
<fieldset>
<input type="submit" value="Login" />
</fieldset>
</form>
Also you may like to see this post:
https://moodle.org/mod/forum/discuss.php?d=26629
