My favourite icon them is Oxygen

Beautiful Old-School look (3D-like look instead of flat design)
Support for scalable icons svg
Lots of icons for desktop applications (actions, mime types, places and so on)
LGPL 3 (or later) license
g-zip compressed icons svgz
Ability to convert svg icons to png

This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 3 of the License, or (at your option) any later version.

I believe it’s perfect especially in your baby steps in application programming. You can use it together with oxygen-sounds and limit your initial assets work only to your custom branding.