I would start on a top-down game, with an up direction. Everything is flat--ie you can't get behind the trees. Once you have a simple one working, put in a few more complex things. If you are new I would start simple and gradually work up.
Some isometric games are really just flat with 2.5 D graphics and a fixed up direction. There are ways to sort in in the z direction by laying tiles down in y order--you can learn about that later. There are also ways to make blocking terrain partly see through. But to start just do a simple top down game.
yours
winkr7