A mood journal can begin with three details: what you feel, when it started, and what was happening. You do not have to explain everything right away.
Over time, review your notes for patterns. Perhaps certain activities bring calm or some situations leave you drained. Use that information to know yourself better, without treating every emotion as a problem to solve.
Every emotion deserves space. If a situation feels hard to manage, seeking support from someone you trust or a professional is also a way to care for yourself.