Wearing is just like makeup. You need to hide your shortcomings and show off your merits. Do not go the other way around, even if you like a dress so much. But it can exemplify your already very big butt, give it up!
The key to look more passable by wearing clothes is to look feminine. If it's summer, you can wear clothes that would expose more flesh. Such as low-cut shirts, shoulder vests, camisoles and something like that. If you have very nice boobs, you should definitely wear low-cut dresses or clothes. This is a major advantage that shouldn't be hided away. But you have to be careful with the limit. Do not go too much, or you will get much attention and there's possibility that you may get clocked for it. The public's eyes are very harsh and sensitive. If it's winter, it will be much harder to show off your body. But if you think cold can't stop you. You should feel free to wear whatever you want. For most transgender people, keeping warm is more important than beauty. You can only choose clothes that may seem more feminine over those which cannot. Take color for an example, do not choose boy color clothes. Try to choose some colors that presents femininity, such as light pink, light purple, or just white will be okay. You can also wear dresses. The choices are plenty.
If you have short neck, you can wear some off-shoulder clothes and dresses. They can show your neck and shoulder. It can lengthen your neck visually. If your butt is super big and you think it is a shortcoming and want to hide it away, you should stay away from tight dresses or tight pants. Try t wear some loose dresses, which can hide it very well.
Actually, wearing can be very easy. Do not afraid to try and make mistakes. You will learn better from mistakes.