Well that really depends on your preference; If you like to use vector software such as inkscape which will allow to design like if you were to be using Adobe illustrator. Then the other route would be to use a raster based software such as the GIMP which allows you to design like if you were to be using Adobe Photoshop.
Vector = Lines and Curves
Scales better if you want to later on resize your image but only works well when creating original works without any photos.
Raster = Dots per inch or DPI
Mostly used for modifying photos or creating digital art work with photos. This works great if your artwork will make use of heavy photo editing/manipulation.
Note: In inkscape you can import raster images and add some effects and export your designs as raster images with the .png file extension. inkscape can also export or save your designs as a PDF file.