How should I build my memory?

I finished junior year (11th grade) and took some pretty difficult classes. I got bad grades (from an Asian perspective) and I noticed that I had difficulty memorizing information or sometimes just understanding it. I usually would study for tests the night before starting at like 8 pm until 3 am because of all my other homework and catching up on some sleep. Still, I couldn't memorize a lot of stuff before the tests. How should I prepare or improve for next year? Thanks!