I think you did a good job. I like your base color choices. I think that the shape of your land is good. I really like the frame of your map but I do not like that the top of your island is overlapping the frame. I think the water border around your islands needs another layer. The dark blue contrast with the light blue to much, I think you should add a second slightly darker blue highlight. The mountains at the top of the map look out of place the land around them needs to have a colder feel to help the mountains seem more natural. As for trees I think you should find a style that you like and try to build off of that at first then you can continue to create a more unique style of your own. I am really looking forward to seeing this map continue to evolve as you learn more, it is easy to learn here... Good work so far.