Supposing you have uploaded the Drupal codebase on htdocs, you have to:

1. Verify that htdocs/.htaccess files is present
2. Modify htdocs/.htaccess:
   1. uncomment the RewriteBase line and set it to:
      RewriteBase /
3. Run the Clean URL test on admin/settings/clean-urls and then Enable clean URLs
4. Job done!

See also:

• Tophost Clean URLs FAQ
• Clean URL con Tophost