Rapid is much more helpful for learning and improvement. Blitz and bullet is mostly pattern recognition and easier with experience.
I assume you know the rules and basics of chess. I recommend checking out this blog post I wrote years ago and it also has many helpful links at the bottom of the blog post.
https://www.chess.com/blog/KeSetoKaiba/opening-principles-again
If you aren't so sure with all the rules and basics of chess, then this link will help you instead:
https://www.chess.com/learn-how-to-play-chess
Any tips on how I could get better?