I join SoundsGreat and other in welcoming you to this forum.
There is one great advantage of converting a roof top party room to a HT room. You have to spend less on liquor, cigarette and other party needs.
If you starting from scratch in converting a room to be suitable for HT, you have lost of advantages. You can literally start with a clean slate and modify your room to have excellent acoustics for music and video.
Some of the immediate steps you can take are the following.
1. Take a piece of paper and not the following:
a) Your interest in music. This will help you decide CD Transport, amplification, and speaker types you can set up.
b) Your interest in movies. If your interest is high, this will help you decide the type and size of your screen, and the kind of HT system you can set up.
c) Your budget
d) You time line.
Once you have these noted down, you can move forward in an organised manner to set up a wonderful HT room.