Why is my hard drive still full after deleting files? I'm new to Mac. I have deleted many files from my applications folder, downloads, music, etc. to make clean Mountain Lion installation.

After installing Mountain Lion I can still see in
 About This Mac &gt; More info &gt; Storage
That my hard drive has 2GB+ of music files and 12GB+ of other files.

How to open my hard drive to see all files located on it and delete them e.g. like on Windows?
You also need to empty the Trash. Right click on the Trash icon (far right of the dock) and choose Empty Trash :)

To view your hard drive, open a new Finder window. Then in the menu, click Go → Home to see your user directory. Or choose Computer to see your whole hard drive.