To do so you need to install the following plugin below
http://wordpress.org/extend/plugins/role-scoper/
Then first, you have to go into Category Restrictions and restrict all categories for an entire role (in my case, Post Author), then go into Category Roles, select a specific user and select the category in which you want to give them permission to create a new post.
This way you will be able to force a certain user to post in a certain category which is necessary in blogging.