Get a jewelers screwdriver and pop open the back at the crown and look and see. In the case you have lost the old battery, you will need the instructions from tour particular watch from Fossil, Call or e-mail them.
You are going to have to remove the battery to find out which one you need. For instructions on how to do this see: http://www.mademan.com/mm/fossil-watch-battery-replacement-guide.html. You can buy the battery at: http://shop.batterymart.com/search#w=fossil%20ch2599%20watch%20battery. Replacing a Fossil battery is delicate and tricky. Fossil recommends that you send it to a shop that specializes in Fossil watch battery replacements.
Take Watch to Watch shop for them to fit new battery .. As you don't have original battery in your watch, you would not what type to buy if you wanted to fit it yourself.
I have a Fossil and a Nixon watch that have very similar backs. Usually when it has those grooves all the way around, the back just screws off. I know they make some special tools to twist it, but I had luck with gently using some needle nose plyers.
Hope that helps and good luck.