You cannot start without a template, as Joomla requires one for the frontend to function properly. If you are looking to build a page from scratch without inheriting the styling from an installed template, you may consider using Joomla’s default template, Cassiopeia. This ensures a clean starting point without additional design elements interfering with your layout. However, if you wish to override the template while keeping your existing setup, you will need to explore options such as using a blank page layout in SP Page Builder, disabling template-specific styles, or applying custom overrides to achieve the desired look.