Qualifications
Let's start with the educational requirements. First, you'll need a bachelor's degree in civil engineering or a civil engineering specialty. Your degree will take four years to complete, and will include courses in math, engineering mechanics, statistics, and fluid dynamics, among others. You'll find yourself in a classroom, but will also have to slog through field work and lab exercises.
Important note: Your civil engineering program must be approved by ABET (Accreditation Board for Engineering and Technology) (source). You'll need to complete an ABET-accredited program to become licensed; and you can't work as a professional engineer without first being licensed. See how important it is?
Now, let's assume you've gotten your bachelor's degree. If you plan to land a managerial position (with a climate-controlled office), you'll need a master's degree and considerable on-the-job experience. You'll also need to be licensed before you can kick back in that manager's chair.
Next, we'll talk about licensing. If you plan to market your services to the general public, you must be licensed in all fifty states and Washington, DC. Why? You're directing other engineers, civil engineering technicians, and other team members to complete a project. You must demonstrate considerable competence to perform those tasks.
Before you get too far along the licensing path, you must pass the Fundamentals of Engineering Exam. Let's say you do that, and you also meet your state's specific requirements. That means you're a Civil Engineering Intern. Now you get to amass more experience, pass more tests, and complete other requirements somebody pulls out of a hat.
Once you've done all that, though, you'll earned your Civil Engineering Professional stripes. Congrats, champ.