There are many things to learn. If you are computer literate then you probably know several of the basics already. Here are a few of the areas to learn:

Scaling: Resize by percentages or by the desired dimensions (measurements), Reduce, Enlarge;
Maintain aspect ratio, Symmetrical scaling, or not;
Non-symmetrical scaling, Change only height -- or Change the width -- or both, but not in the original proportions (not maintaining aspect ratio, not symmetrical);
Resample in terms of pixel per inch or per centimeter, etc.;
Draw/Paint, fill a defined area with color, texture, or percentage screen values;
Modify images/Photos, color "correction" or changes;
Use different formats for saving graphics for differing purposes:
GIF - Drawings with layers, transparencies, can be animated;
JPEG - Photo images are great for the internet;
PNG - Photo images with layers, transparencies;