Welcome to Osu! Galleia! First off, I'm going to tell you something everybody tells you. Play more. Now with that out of the way, make sure you feel comfortable with the way your playing. If you feel comfortable playing with just mouse, that's fine. What I found helpful for me was early on I switched to mouse and keyboard, which is basically the same as mouse only, but instead, you're splitting up the tasks of playing into two separate actions, which I feel is easier to control rather than to rely on only my right/left hand to play. Oh, by the way, mouse and keyboard is just when you use the mouse of aiming, which doesn't include clicking, so make sure if you're going on this route, you don't click (unless you wish to, which is also completely fine) and to use your keyboard instead of clicking. The standard buttons for keyboard are z and x, but you can change those in the options if you desire.
Now about the beatmaps, you should just play maps that give you a bit of a challenge. Not so difficult that you fail within 10 seconds of playing, but not too easy that you can pass it without breaking a sweat. When beginning, you should try to play maps that you can get rank B or C on, then as you get better, increase the difficulty of the map to which you can still get B's or C's. In my opinion, I found that playing with this method works for getting my speed better, but you cannot forget about accuracy. If you can pass really fast maps, good for you! However, without good accuracy, your scores will just be overlooked and can be easily taken from you. So what I would recommend is that occasionally, you play for A's or S's, but never play for SS's. Getting high S's is just fine, but continually playing a single map just for the SS is completely useless. But if you do not wish to follow this advice, it's completely fine. I'm not exactly a pro at this game, but I can at least give you some advice.
If you feel that you aren't improving, just make sure your having fun. If you stress yourself out just over a simple video game, then that completely defeats the purpose of playing, doesn't it? So make sure you have fun, because there's no point of playing if you're just going to rage about it.
"SS = 100% accuracy
S = Over 90% 300s, less than 1% 50s and no misses.
A = Over 80% 300s and no misses OR over 90% 300s.
B = Over 70% 300s and no misses OR over 80% 300s.
C = Over 60% 300s.
D = Anything else."
-From the Osu! Standard Introduction
