I've been where you are. Here is how to determine your size: Measure yourself. First is the bust measurement. Put the tape measure just under your bewbs, and keep it snug around your ribcage. If you get an odd number round up to the next even number. For example, I measured myself as a 37, so that makes me a 38. This is your bra band size. I just had to see how this actually worked, so I bought bras in both 36B and 38B, and the 38 is the one that fits. The 36 is too small and looks awful. Second, waist. Measure yourself at the smallest point, which for women is usually an inch or so above the belly button. This is called the "natural waist." For example, I am a 33 in men's pants, but I am a 30 in women's pants, and that is my waist measurement in women's clothing. Finally, hips, and this is the easiest part. Just measure your hips at the widest point. I think I am about a 37 there as well.
Now, with your measurements in hand, you are ready to navigate all those online size charts where they translate numbers into women's sizes. I usually translate to a junior miss (odd number sizes) size 11, or size large, both top and bottom. In misses sizes I usually am a size 8 or 10 down below, and mostly a 12 up top, because I still have some shoulders going on. For tops I am always a size large, and for pants and jeans I can be 8, 9, 10, or 11 depending on the maker, but I am always a 30 waist. If I buy a women's 31 I have to wear a belt. But wait, there's more! On anything that has a low waist or hipster waistline, I have to go a little smaller, so my hipster shorts are usually size 8 or 9 or 29 inch waist.
This is a learning experience. Have fun with it!