Hi and welcome.
Using premade brushes is a good choice for absolute beginners and StarRaven set is just amazing. When you will be more experienced with layout and composition of maps, making your own brushes will give your works much more uniqueness and personality, and you will feel far much rewarded.
As for this map: the most visible problem, in my opinion, are overlapping trees. Try not to overlap them, and/or find a way to “stack” them without overlapping; for this and other problems that you ask about (e.g. how to create your own brushes) take a look at the tutorial section, it’s really full of tips and suggestions.
I also have some doubt on the hydrography of the map: the two rivers that run parallel to the coastline, instead of ending up on the shortest route to the sea, must be justified by special features of the drainage basin that did not appear so obvious (at least for the river to the west ). Even for every river problems, take a look to the tutorial section, there’s plenty of information on this subject there.
For the layout, adding a border and detailing a little bit the map with a legend, could be generally an improvement.
Another piece of advice I can give you is to look carefully at the work of others, there are some fine artists here, you can get much inspiration from them.