Realistically you can make money without any investment, 15$ isn't much so if you are going to use it, use it wisely.
I would recommend hosting and a domain name, then create some sort of niche website or most likely use it to promote a content locker or CPA offer.
I am assuming you are just starting out, so you need to figure out a game plan first. You could also buy some cheap articles and services, but really all I do with 15 dollars is buy a domain name for one of my campaigns.