Well, as you're 18 you will probably see more changes than others that transition later. On HRT you will look "more" like an hourglass for sure but it's different for everyone. If you begin your transition while having a skinny body you will see the changes faster as the "new" fat will pack already on the good spots but with time the existing one will replace anyway. I would not suggest to "waste" time for losing weight, if you can start your HRT just start it the faster you can if you know you want it. If you still have time before starting it then yes losing weight can be a good idea.
It's easier to lose weight before HRT? Well yes and no. You will be able to lose weight while eating more before HRT but with the same deficit you will lose as many while on HRT. The point is that you can easily stay at BMR calorie count without much problems before but you can slow down your progress if you starve yourself while on HRT.
In my case my body fat was pretty weird... Every part of my body had a up and down in fat at a time then everything got stable. They all looked too thick and too thin at a moment for the rest of my body it was pretty weird. Well I was packing fat in my belly before hrt (not many but enought to never show my abs no matter how thin I was) and now my belly always stay flat, I likely don't pack any fat here anymore as I always seem to see a kind of light 6-pack.
