Part of your first milestone is a document outlining the features you want to implement on your site.
As I strongly recommended, The more you brainstorm features for your site now, the less questions and headaches you will have down the road.
Create a document in google docs (share with melissacabral(at)gmail.com). Write a list of all the features you envision in your project. Try to break large features into smaller chunks. Even if you do not know how to implement a feature, write it down! Brainstorming is not a commitment, it is an organizing tool.
Here is an example brainstorming session for a social network for dog lovers. Notice how i have roughly identified features that are “build now” and “build later.” Items to Build now will make up the Minimum Viable Product of your project.