Like Tom said, at zero you'll get the most value out of direct outreach to customers you believe are your target avatar. Ideally you'd do this before you build much software, in fact.
The best way to get the process rolling is to request feedback or an interview-like meeting with them to understand their business and problems, maybe doing a light demo of what you're building. Get them involved early and keep them updated as you progress, and when it comes time to ask them to try it out/sign up and pay you, you will have a warm lead and relationship.
Growth marketing is grunt work until you're scaling. Also happy to chat specifics if you'd like to set up a call.
