The best songs for improvement at your level are genuinely just whatever your favourite songs are, played on normal, or hard if you want to push yourself. You don't need to worry about farming, or improving specific skills etc. until you reach ~1000pp or so and you've gained a solid grasp on reading. The best way to learn the game's mechanics is to have fun!
Just make sure you stick to
ranked beatmaps. osu! essentially has a quality control system where (in layman's terms) only maps judged worthy by Beatmap Nominators can have a leaderboard. This way, you can tell that if a map doesn't have a leaderboard, that it
usually will vary in quality. That's the only thing I'd say you need to worry about
