Do one thing at a time until you understand it. Some of the areas are:
Models (textures, mesh, particles, etc.)
Entities (abilities, artifacts, planet types, ship types, etc.)
Textures (the GUI, Icons, etc., etc.
Manifests, String and Definition Files, etc. etc. (where additions and changes in other areas are cited).
Galaxy Maps (Galaxy Forge map maker, templates, etc. etc.)
There is more than this but this is an easy to begin description of the different areas you might change-mod-create in. A mod can be some or all of these things. Just depends on how ambitious you are.