Where to start with such a question… Well, first off, you should produce some sort of concept art that determines the overall visual style of your island. This in itself will help to answer a lot of your questions. Like the basic color scheme of your vegetation assets and materials. Or the lighting and ambience. Also, this will help you decide what sort of vegetation you want to use (trees, bushes, grass etc…). However, if you can’t produce concept art by yourself, then do this:
Gather reference images either on the internet or go outside with a camera. Try to find 20 or 30 images that depict a visual style, color, placement, lighting and ambience that you like. Take these images and produce a collage. This will be your style guide for the map/world you are creating. From here you can begin to find out what sort of assets and materials you will need. Use it as a guide to influence the way your map looks in terms of visual style.
Aside from the style guide, you will need to watch some tutorials on creating assets, or buy some. Megascans for example has some really nice materials and vegetation for a low price. They don’t have trees though (not yet). Make sure to watch their Youtube tutorials for using the assets in UE4 before buying though. That way you can be sure you want to use them before spending money.
For trees you can look into SpeedTree or Onyx (only two I know of). There is also some really good tutorials on the Gnomon Workshop (https://www.thegnomonworkshop.com/) for creating trees and vegetation. All of these things cost money though. Otherwise I’m sure you can find some free stuff online.
Also, when you get far enough, don’t be afraid to post your work on sites like Polycount.com or Artstation for peer review. There you can get some great feedback on how to improve your work.