Ahh I remember when I first started trying to learn python scripts for QGIS almost a year ago (and I'm still quite bad at it!). The documentation I followed to some extent was the PyQGIS Cookbook which offers help in such things as describing how to do simple tasks through the Python Console rather than through interface.
Another huge help I found was in an obsolete feature of the latest Processing plugins, (version 2.2.0-2 had this feature which can still be downloaded) which allowed users to use the graphical modeler and be able to export the model as a Python Script. This gets you familiar with calling the vast array of tools and the parameters they require.
Finally, there's plenty of questions asked here in GIS:SE which involve python scripts. This in itself is a massive learning resource not only for reading other posts but you can also ask for help from wonderful experts (not me!).