first, where do you get songs? I just got the game but almost every song I search is not in it. Also, I read that people play with keyboard and mouse. What do you use keyboard for? I thought this game was all mouse.
And people use the keyboard to click the circles, just using the mouse to aim instead. (people also use drawing tablets instead of a mouse, taking out the mouse completely.)
The default keys for that are ZX (I think), but you can remap them in the settings.
I've used this as my main way of getting maps (other than through online multiplayer) because I am not the musical type. But there are tons of great collections of packs that I would suggest! Try out the Internet Pack for example. There is some funny stuff there!
As for the keyboard you can use the Z and X keys but they can be remapped under "Keyboard" in the Settings. They're right at the top of the list as well, so you can easily change them.