Ok, so for your terrain, I would create a procedural terrain In the terrain edito, under the sphere that reprsents the altitude production function, click the load button, and load one of the altitude functions preset from the visual browser. There are preset desert functions, just play a bit with the function scale in the tarrin editor to get something you like. For the material, I'd recommend using layered materials for added realism. For the base layer, I'd recommnd the sand dunes materials from the material layers collection. Then add some flat sand on top of it. Vary the altitude:slope of the the second layer and add fuzziness for all parameters you change, so materials will blend more naturally together. You' now have your desert. For the trees, you have 2 basic options: ecosystem painter, where you choose the species you want to populate your terrain with, and then paint them wherever you want. Great powerful tool for precise placemnt of your instances.
Or you can use the ecosystem material option, where the eco depends on the underlying material. Here you can't place trees as you like, it's more random, but you can drive the density with a function, or even a bitmap ( create a b&w bitmap in paint program with your name on it, and use it as a density function in Vue, so your name will be written in palm trees!), you can also act upon the rotation and orientation of the instances with this method, by using the corresponding tabs in the editor.
Now about editing plants, I suggest you have a look at the documentation pdf that come with Vue, there is a big chapter about this. Too long to explain here.