how can I provide Role based access to Blog app?

Question

I am confused in how can I develop user roles to the Blog application?

1. I have tried to give roles in user model itself [Admin, user, author]
2. But now I am getting confuse, how can user have aceess of all blogs to READ.
3. If my user wants to create a blog, then his role is changing at that point of time form "user" to "author" of a prticular blog.
4. And "admin" have only accees to delete blogs. So can I give DELETE access to my "user" or "author"?