The icon collection is distributed under the Creative Commons license agreement, which allows you to use the images in both personal and commercial projects. When using this images you should always indicate Aha-Soft as the author of the icons and provide a link to aha-soft.com.
There are many different ways to get icons for your application. One of them is to design those icons yourself using one of the widely available icon-editing programs. But that could be a hard task, if your drawing skills are low or not present at all. The other way is to use a search engine and get the icons free of charge from someone, who does have some drawing skill and therefore, managed to create the icons and put them on the net. There is only one problem with those icons - their quality.