Discuss and debate their relative importance. Then, hold a vote to decide which you want to tackle. Now that you have decided on a problem, it's time to think of some solutions. First, you should learn more about the problem you want to work on. For example, littering is caused by careless people.
Crowded schools are a government problem.
But it helps to have as many people as possible working together. Try to get friends, family, and classmates involved. Also, keep in mind that some problems took a long time to develop. They may also take a while to solve, but every journey begins with a single step. It is good to come up with a plan first.