First thing you have to do is post content that would interest a large number of people. After you've made a few posts to give people a taste of what your page will be about, look around Tumblr and find some good people to follow. When you follow people, you have about 50/50 chance they will follow you back. All depends on whether or not your page interests them.
After that, you then tell your friends about it. If you have a Facebook and your Tumblr page is appropriate, maybe let some people from there know about your Tumblr as well. If you tell people you know about your page, encourage them to tell their friends. Take that audience, and keep branching out from there.
I'll send your link to my friend, i can promise she'll follow :) (she's please-please-babylemonade)
thanks, thats awesome (: