The quickest and simplest way to create new parts is to use Fritzing's generic parts and define their properties in the Inspector.
These include ICs, resistors, pin headers and the Mystery Part.
For example, the name and number of pins of the IC part can be defined in the pulldown menu inside the Inspector:
and the result is...
It might be important for you to define some more part specifications (properties, connector names, etc.) and you would probably also want to save your part. To do so, select your part and edit it using the Part Editor (Part>Edit...). For more information, please check the following pages: