Copyright registration is easy to do online with the U.S. Copyright Office’s eCO Registration System. A complete copyright registration consists of three elements, all of which can be done online: submitting the application, paying the fee, and submitting a copy of the work to be registered. As of this writing, the fee for a single photograph is $35, or you can register multiple photographs at one time for a total fee of $55. You can pay via credit card or electronic check.
The application can be filled out by answering a series of prompts, and the Copyright Office makes a tutorial (pdf) available to walk you through it. The application is longer than you might expect, but it is fairly easy to simply follow the prompts, and you may save your application at any point if need be, so that you can finish it later. If it is your first registration, then you will create an account. You will choose a Single application if you are registering a single photograph, or the Standard application to register a group of photographs. Note that in one Standard application, you may register either a group of unpublished photographs, or a group of published photographs that were all published in the same calendar year, but published and unpublished photos may not be registered together, and photos that were published separately in different years may not be registered together. 
You will be prompted to choose the type of work (for photographs you will choose Visual Arts), and the title of the work. If you wish to enter additional titles, you may select New and enter a new title. However, if you are registering a collection of photographs, it is not necessary to enter the title of each photograph; you may simply enter a title for the collection. You will then be asked whether the work is published or unpublished, the year of completion, and (for published works) the date of first publication. You will identify the author of the work (which may be a pseudonym) and the copyright claimant, and the contact information for rights and permissions, correspondence, and mailing of the copyright registration certificate. You may then review the application, certify it, and pay the fee.
After you have finished the application and paid the fee, you still must submit a copy of your work. Unpublished work and work that has only been published electronically may be uploaded online. If a hard copy is required, you will create a shipping slip and use it to send a hard copy to the Copyright Office by mail.
Once a copy of your work has been submitted, you will be able to select your submission and see the message, “Claim submission completed; no further action required.” As of this writing, the Copyright Office’s processing time for online applications is 8 months (or 13 months if you apply by mail). However, once the registration is processed, the effective date will be the date that the Copyright Office received all of the required elements — application, fee, and deposit of the works — in acceptable form. If you need for the process to be completed faster so that you can take legal action, you may request “special handling,” which can result in the registration process being completed in 5-10 days, for a fee of $800.
For a more detailed, page-by-page explanation on registering your copyright online, see photographer Tom O. Scott’s how-to guide.
For more information about Copyright Office practices, consult the Compendium of Copyright Office Practices.