The main key to getting better quickly at any rhythm game is to work on speed. Accuracy usually comes within time as you are training. Also, ignore improving your osu rank. I found that to be just a big waste of time. I would work on that AFTER you get really good to reduce whoring. There are some other elements to that but I do not feel like writing up a huge essay about all that so...practice! practice! practice! Trust me, I have over 6+ years of rhythm gaming experience.