Welcome to the forum.
As to your questions, I assume you have done some reading. When I went through that stage, I started with Woodland Scenics plaster cloth over crumpled newspaper and got some good mountains, but they are VERY difficult to plant trees on.
I then discovered styrofoam insulation in 1 and 2 inch thicknesses and that is now my staple. There are many threads on that, and my pics(see signiture) show some of what I did. Carving foam is easy and fun.
For the desert I used foam and had a good time making cactus.
More sprcific questions will get you lots of answers. Good luck
You can also use aluminum window screen hot glued to wooden support elements along your layout, and then cover the aluminum with a mixture of plaster of paris, fine vermiculite or sawdust, and portland cement. I used the material in the ratios recommended by Joe Fugate, but I also added some masonary dye to the mix and then added water. You ladle or spoon the goop onto the screen and spread it around.
Mind you, the stuff is heavy when you mix it with the water, and it doesn't like to be manipulated for more than about 3-5 minutes, depending on the amount of water. So, two things to remember: you can use crumpled newspaper below the aluminum screen for support, and even crumple the newspaper into plastic grocery bags and tie them up tightly. The result is a more robust support than merely the newspaper. Secondly, if you find that you are getting into stiff goop and it doesn't want to spread, simply add a couple of tablespoons of water and stir, then get it on the screen as soon as you can.
It would be a wise investment to place two layers of newspaper, or else plastic sheeting, directly below any places where you either mix the stuff, add water and remix, or scoop it up and place it on the layout.
"How would you construct mountains and a small desert?"
Foam or chicken wire, with odd pieces of foam or bunched up newspaper taped to the chicken wire. Then cover it all with plaster cloth. I get the plaster cloth at an arts and crafts store, much cheaper than WS plaster cloth. After the plaster is dried, apply a thin layer of sheet rock joint compound, either mix it yourself or buy the premixed kind. You might need two layers of compound. The thinner the better or it will crack, but you can always patch up the cracks. finish off with a coat of paint and voila, mountains, ridges, etc.
For desert I have some very fine mason's sand that I sift directly onto wet paint or sprinkled onto a very thin layer of white glue.
Bear "It's all about having fun."
Awe Cactus. That was the question. I purchased a few from a man at the Club in Phoenix that were nice , but expensive. I do not know if he still does that. I tried two comercial types and they were bad. I tried some from Pipe cleaners and they were worse.I am now making mine with this Plastic clay you get at a craft store (several brands). I roll it into shapes about the right thickness, then roll it over a board with grooves carved in it to simulate the grooved look, cut and bend the shapes, stick them together, stick a pin in the bottom and stick it in a piece of wood. When I have a few, they go in the oven as per directions. About 3 out of 4 are useable. They cost about 2 cents apiece. You can vary the size to work forced perspective up the mountainside.
This is another place where expermentation is better than instruction. Show us your results as we are all learning this together..
"Well the desert I am modeling after does not have any cacti but has spruce trees, here or there. It is known as the worlds smallest desert in Carcross Yukon."
There ain't no desert like that down here in Arizona.