There's a nice little thread that happens to be all about both how to get better (hint: it starts with 'play', and ends with 'more') as well as what the process of learning to play really looks like.
The first thing to do is simply play until you no longer need to consciously think about hitting the right key. Until this happens, you're always going to struggle. The amount of time it takes to get through this can vary between players a lot. Some people might be able to get through this in a few hours, while others might take days or weeks of playing before they really learn to respond automatically.
Space bar is always going to stand out. It's pretty much everyone's weakest key. It's certainly mine.
I have one issue with what Shiro said:
Shiro wrote:
Don't try to play things that are clearly above your level.
Be careful saying this. One of the most important things about improving in mania is actually to break out of your comfort zone. Many people mistakenly believe that unless they get high As or S's they can't really 'play' something. In mania, you should make sure you are playing songs over a wide range of difficulty, based on the score you can achieve on them. You should make sure to play songs you can C or B as well as songs you can A and S. Playing songs you can only manage a C or a B on will help you improve quicker than playing only things you can A or S. Playing songs you can A or S will primarily help you improve your accuracy, but not your overall ability to play harder things, whereas focusing on songs you can only B or C is going to help you learn to play harder patterns much faster, but will not improve your accuracy on things you're already relatively good at nearly as quickly. The best approach is probably to keep a good balance between these.
Shiro wrote:
Overall, play mania-specific diffs rather than converted ones, too.
This is basically necessary. Mania specific maps have very different patterns than autoconverts. Autoconverts do not pay attention to things such as layering multiple rhythms at once, or following pitch properly. Patterns created with layering often require much more individual finger dexterity because you may find one hand playing a completely different rhythm than the other hand, which can be very difficult at first. Patterns which follow the pitch are actually easier to understand than autoconvert patterns, but can sometimes be a bit tricky when they are mixed with other patterns.