My general database design steps are three:
1: Identify Entities (User, Picture, Comment, etc) [These will be your main tables]
2: Identify Properties of the Entities (User’s Name, Email, etc) [These will be fields in your tables]
3: Identify Relationships Between Entities (A Comment is posted by a User) [These will be Foreign Keys and/or Join Tables]
Or more wordily: What are you storing information about, what information are you storing about them, and what ties do they have to each other.