This is where you need to be careful, as if you do this wrong, you can lose all of your files. 

You will be confronted with three options as you can see in the screenshot below. Unless you want to JUST have elementary on your computer, choose the first option, "Install elementary alongside other operating systems".

Then elementary will give you a slider with all of your operating systems on it. Simply drag the bar in the middle to allocate each OS a set amount of space.

Then click "Install Now"