Survery: Does anobody know how to put an ad on tumblr?

I want an ad so I can save up money for a nikon d3100. My parents have no money. But I'm under 13. I tried google ad words but that made no sense. Is there another website where I can get a free ad and the money get sent by mail? Please someone tell me. And there are people on tumblr under 18 who have ads.

You practice on a banana…

You can't. Plus, you need thousands and thousands of followers for them to accept you. Otherwise, you're really not getting them any business.