Influencing policy starts at the local level.
For young people all over the world, local governments are first points of contact for important issues like education, labor, and LGBTQ+ rights. To have an influence at the local level, you have to know what you're dealing with. This resource, authored by the fine people at the International City/County Management Association, contains descriptions of a wide array of different types of local structures, from Townships to Special Districts.
Many resources on this topic revolve around towns and counties in the United States. This resource is especially useful because it takes an international perspective in its offerings.