Welcome to the guild first of all There definitely is a lot to take in here. The most obvious answer of where to start is the tutorial section I guess. Tutorials/How-To
There area a lot of different approaches you can take and they also all depend on what your end goal is. My advice is to just start with a tutorial and follow it and start learning My other advice is to try to anchor your maps with a bit of realism where possible as that helps with believability regardless of the style. A prime newbie mistake, for example, is not making sure your rivers flow one way into larger bodies of water. Except for delta situations, rivers in the real world never do this.
Anyways, try making some maps and post them on the forum to get feedback and grow as a cartographer from there