This program can help you out a lot if you can't read music sheets.
(This is the easy way out) The first shortcut. :)
First, go to http://www.synthesiagame.com/getTheGame.aspx To download the installer file, choose your operating system.
Install with the recommended settings, If for some reason it says the program wasn't installed correctly, just install it again.
Once you have it installed, Open Synthesia where you stored it.
There should be pre-installed MIDI files if you used the recommend settings.
Please note you can download additional MIDI files. Just look it up on Google.
Here's how your search should look like. "*Song Name* MIDI". Save it in the "Synthesia Music" Folder. Should be located in your Documents.
You can slow down by pressing the "down arrow"
Speed up by pressing the "up arrow"
Pause by pressing the "Space Bar"