is this what you want to do?
then you need to go to your website then right mouse click and then click view source. then search for user-login-form you need to copy all the code where the form starts and ends onto the block--user--login.tpl.php file
for example block--user--login.tpl.php contains:
<form method="post" id="user-login-form" accept-charset="UTF-8"><div><div class="form-item form-type-textfield form-item-name">
<label for="edit-name">I just edited this, woo<span class="form-required" title="This field is required.">*</span></label>
<input type="text" id="edit-name" name="name" value="" size="15" maxlength="60" class="form-text required" />
</div>
<div class="form-item form-type-password form-item-pass">
<label for="edit-pass">Password <span class="form-required" title="This field is required.">*</span></label>
<input type="password" id="edit-pass" name="pass" size="15" maxlength="128" class="form-text required" />
</div>
<div class="item-list"><ul><li class="first"><a href="/user/register" title="Create a new user account.">Create new account</a></li>
<li class="last"><a href="/user/password" title="Request new password via e-mail.">Request new password</a></li>
</ul></div><input type="hidden" name="form_build_id" value="form-_elpq-7B19Scnbgk13TZtp9YWCcANc-rDYCWcHPuihI" />
<input type="hidden" name="form_id" value="user_login_block" />
<div class="form-actions form-wrapper" id="edit-actions"><input type="submit" id="edit-submit" name="op" value="Log in" class="form-submit" /></div></div></form> </div>
</div>
</div>
Pros: You don't need to know PHP nor how to create a custom module. (The noobie approach).
Con: If you change theme's you will need to make a copy of this template and put it in your new theme's template folder.
Counter the con: There might be some other (views, blocks, nodes?) templates that also need to be moved if you change themes, since this is in the /sites/all/themes/yourtheme/templates
folder, you will likely not forget to move it with the others? Therefore, this approach is not as bad as people may think.
Don't get me wrong, the accepted answer is the better approach, if you have the knowledge that is.