Well, if you don't have much fat to work with, the effects will be more mild regarding fat distribution. However, muscle reduction is sometimes really impressive. Through fat redistribution, muscle loss and cartillage/tendon/whatever reshaping (as well as bone changes if you're still under 25 and get lucky), your body shape will change towards a more female shape. You should have less volume in the waist, and more in the hips, bosoms and thighs. The upper body will probably slim down the most.
If you want to maximise effects, given that you're quite skinny, gaining a little bit of weight after starting HRT might be a good way to make changes more apparent, if you're able to. If you can do a little cardio exercise whilst doing that, it's probably best, since it helps the fat you have in male spots burn.
For a rounder butt, you might have to do some exercising, though. Muscles tend to slim down on HRT, and I can't guarantee that your butt fat is going to look round. The "round, high and sexy" butt that people want is actually muscle for the most part, I believe. So you'll need hard work. You could look into brazilian butt lifts.