Understanding overall database design

Understanding overall database design

I have an interest in understanding database design, but I am unsure of
where to start or what I should be paying attention to.
For instance, if I am creating an application, let's use a blog for
simplicity, I know that I will need data. I know that the data I will need
is posts, comments, users, categories, images, etc. I know this "data"
will come in the form of models or tables.
What I have a hard time understanding is when I need a separate table vs.
adding another column in an existing table.
For instance, my Posts table may look like:
ID
Title
Author
Category
Image
Body
However, I want to manage the Categories, so I think I should have another
table just for Categories. What about Images? Should that be a separate
table?
I don't know when to break it out, or keep it together.
Can someone point more to some documentation that would help me learn this
concept? Some guidelines or rules per say.
Thanks!