1. Where you're going to put it
2. Will it fit?
My sister and I measured our room, how wide it was and how long. (Do not include the closet when you're measuring.) Then we took some graph paper (http://www.waterproofpaper.com/graph-paper/grid-paper.shtml this link has six different free printouts.) and used one square per foot. So lets say your room is 10 feet wide and 12 feet long (just random numbers that popped into my head). You would cut out a piece of graph paper 10 squares wide and 12 squares long. So you would have a rectangle with 10 squares going to the side and 12 squares going down (I'm doing my best to explain this so it makes sense, if it doesn't then feel free to ask questions in the comments).
Then you're going to measure all the furniture you want in your room, your beds, your dresser, vanity, desk, and any other furniture you might have. It doesn't matter how tall it is since you'll just need to know how much room it will take up on your floor. So measure the bottom of the piece of furniture, width and length, and cut it out on the graph paper, the same way you did for your room.
Once you have all of your pieces of furniture cut out (don't forget to label them) you're going to start arranging them on the piece of graph paper that's representing your room. This is going to tell you how much furniture you can fit in your room and what would be the best way to arrange it.
The next step is to put the furniture into the room. It may take a couple times of rearranging to find the best set up, but you'll find it. Remember, you may have to compromise with this. You may not have room for your keyboard because of your dresser. Or your vanity won't fit because of your nightstand. But maybe you will have room for everything. Just don't shove everything into your room. Remember, you need walking space too : )