Hey there. I'm new to cartography and I am trying to create a fictional world.

I tried to make a map a few times but I am always having trouble deciding on where to put the forests on my map. I gather that they grow along one side of a mountain range, because mountain ranges trap moist air/rainclouds. Also, the soil around rivers tends to be fertile so they can also be along the rivers, but I am always having trouble deciding where exactly along the river. I also have trouble deciding what kind of vegetation there should be around the rivers otherwise. Trees or bushes or just grass? Under what other circumstances is it realistic for there to be forests?

Thanks for your time. I would appreciate it a lot if somebody addressed these points.