Mmmkay. Full review coming right up!
First and most obviously, you use straight lines in your cliffs and symettry in your trees. What you want to go for in nature is
asymmetry, which is
not putting things in a perfect 'L', especially not three identical trees. It's a desert, so you may want to scatter them a bit. If you want an oasis, it's okay to put them a bit closer, just don't overdo it. Moving on, your cliffs are also pretty bad. Straight cliffs are a common problem in inexperienced mappers, and I blame it on the poison that is Poke'mon games. The rule of thumb here is to not place over three of the same tile in a row. Think curvy when making cliffs, and read
this. It's a great tutorial by Acera.
Besides the straight lines in your map, there is the slight lack of ground detail. This sounds nitpicky, but a little variety in your sand tiles makes a big difference. You can also put a few more small, one tile plants if you have custom tiles. You might also put a couple non-hostile critters around the oasis. Moving things make a map seem alive. Again, though, don't overdo it.
Other than that, you have a pretty nice base of a map. It reminds me of my first map with RMXP. Good luck man, I'm around if you need any more help.