If I wanted to make a website similar to an existing one, how do I know that I'm not breaking its copyright?
For example, how do I find this out for a website such as Facebook?
Graphic design is considered as intellectual property and is automatically protected by copyright.
You can't just "copy" a design. However, creating a new one inspired by another one is perfectly legal (with some limitations).
The biggest rule about inspiration is that the new design can't be confused with the existing one.
Want to create a logo with an apple in it? No problem, but the vast majority of the people must not confuse your logo with Apple Inc logo.
You can create a slim blue design that remind the feeling of Facebook, but your users must not believe they are on facebook or that you are affiliated with them.
This discussion is confused.
The form of intellectual property (IP) that is called "copyright" generally covers "works of authorship". Examples are books, music lyrics, photos, longer texts on a webpage, etc.
The IP that is most often used for logos and brand elements would be "trademarks". Examples are registered trademarks for the Nike 'Swoosh' logo, Apple's logo, some corporate slogans, etc.
Lastly, there is a common form of IP called "patents", which in some countries can cover any innovation that is deemed sufficiently novel -- and in other countries patents are more restricted in scope.
A design in its abstract form -- what is known as "look and feel" -- the colors used, the position of main elements, etc -- generally cannot be legally protected.
A very specific design element (such as a logo) or an innovation (such as a way of interacting with a user interface which no-one else has ever thought of) will often be legally protected.
"Graphic design is considered as intellectual property"
True...to a degree... but a layout isn't graphic design, just the elements are. A shape is generic, public domain, just like these words.