Authors acquire the rights in an original work as soon as they fix the work in a tangible form of expression. A fixed form includes standard written or drawn forms as well as those that humans cannot read or interpret directly without the aid of a machine or device. Neither lawmakers nor the courts have defined originality with precision. As a general matter, originality means the author originated a work that contains more creativity than a simple compilation of facts.
You satisfy the fixed format requirement when you write your idea on paper, paint it on a canvas, record it with a camera or perform a host of other common actions to express yourself in a tangible manner. You can test if your work meets the requirement if you copy your idea into a material object that someone can read or perceive visually. Phonorecords are the means by which you can fix sounds into material objects. Statutes broadly define phonorecords to include cassette tapes, CDs, DVDs and vinyl disks.
Copyright laws do not protect you from other people who may copy your ideas. As a practical matter, it is very difficult to determine who may have had an idea first. It is also difficult to establish the specific content of your idea if it is not in a form that someone else can read, hear or see. Putting your unexpressed ideas into tangible form provides a set point from which the time begins to run on your exclusive rights over the work.
Copyright law protects the expression of mundane ideas as thoroughly as it does extraordinary ideas. Requiring authors to express their ideas by putting them into something tangible frees everyone else to generate ideas without the fear that they will need to defend themselves against a lawsuit for infringement simply based on the similarity of ideas. Even after you express an idea in fixed form, the idea itself does not fall under copyright protection, but fixed forms that contain your idea are. Others may take the same idea and use it for their own purposes. Einstein's theory of relativity is an idea and not copyrightable, but papers he wrote about it are copyrightable. Therefore, others can use Einstein's theory and create other copyrightable essays about his theory.