Start in Google Flow (labs.google/fx/tools/flow), not the Gemini chat app — video there currently needs a paid Google AI plan.
The free allowance is small and spent per attempt — and Google is inconsistent about whether you get one. Flow's own page and its credits help article say a signed-in Google account currently gets a small daily allowance of Flow credits, the unit of spend; Flow's "Get started" article lists a paid Google AI plan as a requirement. Check which one your account lands on before planning around it. On that reading, the top-quality Veo setting costs about ten times the cheapest: a handful of short clips a day, or a single top-quality one. Credits don't roll over, and mush costs the same as your hero shot.
You'll pay past the experimenting stage, because ten attempts at one shot is a normal session. Paid Google AI plans run from a low-cost entry tier through a mainstream tier to expensive top tiers; check the plan page, these move.
First step: sign in, pick the cheapest model, and run one description several times. How much a single wording varies is day one's real lesson. You must be 18 or over and in a supported country.