In regards to your query on resources, the books and creators that Turambar pointed out are some of the best that I'm aware of for mapping specific stuff. On the YouTube side, WASD20 has some map-making videos for the beginner-level mapmaker but again there isn't heaps there that I'm aware of which specifically focus on maps (I've toyed around with the idea of having a go at creating videos myself but don't have the available free time right now). I would recommend getting stuck in and making some maps yourself, then posting here for feedback. Competing in our map making challenges can also be a good way to push yourself and is one way I improved my skills. I would also take a look through the finished map thumbnails subpage of the finished maps forum, find art you like from other map makers and try to replicate it or incorporate elements into your own work.

On the side of improving the realism of your maps and world building I've always found this medieval demographics made easy document to be a great resource.

It's great to hear you are so excited about map making though and I look forward to seeing your work on the forums.