In another post, I discussed the low-bar for copyright registration. In short, as long as your work is "fixed" and "original," it can be protected. Even without registration, your work can be copyrighted work, but there are clear advantages to registering your work.
A copyright is available for the expression of a work, but not its underlying ideas. Here is a list of the types of works that can be copyrighted. In order to copyright your work, you need to file it with the Copyright Office. To properly file, you need to fill out the application, make payment, and deposit your work. Some works can be deposited online, and some require hard-copies. Even when a hard-copy is required, the application without deposits of work can be completed online. After filing, it takes a little while before you get the registration certificate. So, prepare for the process to take several months.