I disagree with Redon here. I don't think it's very helpful to have some specific accuracy goal that applies to every map you play. It doesn't promote a healthy attitude towards the game. VSRGs are ultimately not a game about reaching some specific goal. At their core, VSRGs are about challenging yourself to get better. If you look at the game itself (what I mean is ignore things like ranking that aren't directly part of actually playing a beatmap) the goal is very simple: hit as many of the notes as accurately as you can.
In order to get as good as you can at doing this, there are several different skills that you must develop. When you ask questions like 'how do I get better?' most people will say 'play more', some people will give suggestions like Redon gave above that suggest certain tactics in picking what you practice in order to maximize your learning of these skills. But there's something nobody talks about that's far far more important than what songs you play.
The single most important thing you need is the right attitude.
I'll give you some examples of behavior that comes from a bad attitude. When people give up the moment they make a mistake, that's usually because of a bad attitude towards the game (except when someone is specifically trying to get a certain kind of score, such as a full combo or an SS). Mashing keys when you reach a section of the song that is a bit too hard to play completely properly. Getting angry when you fail, etc.
The only goal you should have when you play a beatmap is to play it. That's it. If you try too hard to reach some goal, you will make more mistakes, you will teach yourself bad habits, and you will constantly be trying to judge whether or not you are good enough.
Relax, focus, and just play. Don't think of anything else but the music and the notes. Don't let mistakes bother you.
Next time you play a beatmap, hit shift+tab to hide the score and combo and other counters. Not only does this reduce the number of visual distractions on the screen that can break your focus while playing, but it will make it easier to stop concentrating on how well you're doing and just enjoy playing beatmaps in general.
Now, I'm not saying that you shouldn't do things like try to improve your rank, or set goals for yourself. I'm saying that those things shouldn't be in your mind when you're actually playing. I still give myself goals such as trying to S a map I haven't S'd yet, or to pass songs I haven't passed yet, or just to improve my personal best on a certain map. But I never think about any of this when I'm playing. Most of the time I have no idea how well I'm actually doing on a map. I might have a small idea because maybe I'm making more combo breaks than usual, or i'm simply playing better that day, but I never know for sure until the song is over and I've checked the score screen.
So, to sum everything up:
Relax, don't worry about how you're doing. Enjoy the beatmaps, pick goals, but don't obsess over the little details.
Clear your mind when you play and just try to focus on the beatmap you're playing.
Don't let mistakes and fails bother you.