Pick the free Developer Edition org if you want one Salesforce org to learn and build in for months, and switch to a 30-day trial only when you need to see a paid edition with sample sales data. Trailhead Playgrounds are for badges, scratch orgs are for source-driven development, and sandboxes are never free on their own.
A Salesforce developer account is really a Developer Edition org: a full Salesforce Platform environment with Agentforce and Data 360, no credit card and no end date while you keep logging in. Below you will sign up for one, verify it, find your login URL, keep it alive past the 45-day inactivity rule, and see where the other four free options fit.