This is a seriously bad question. Create a file and rename it to css. Somehow I think this is not what you are truly asking. Perhaps you should read up on css a bit because no one will be able to help you if you don't at least try yourself.
creating css file is very easy just create a blank file and save it with .css extension for an example myname.css. But writhing css is little bit complicated. If you ask any help about any div then you can get many answers from forum but if you want to learn how to write css then you can take a tutorial from you can take from here it's very easy. hope it will help you.