I’m not part of any student government. What can I do outside of school to show I’m a great leader?
You don’t have to be a part of student government to show leadership, as long as you have leadership roles in other places within your extracurricular activities. Being captain of a team, becoming the president or vice president of a club, founding a club, leading in community service activities are all ways you can show adcoms that you are a great leader.
You could suggest to you rec letter writers that they mention your leadership history.