Qualifications
You will need to get a bachelor's degree for most jobs, preferably one in criminology, psychology, or physiology.
You will also need to have gone to a polygraph program accredited by the American Polygraph Association. There, you would receive a certificate stating you have completed a certain amount of hours in the school and have conducted many sample polygraph tests.
The CIA wants its applicants to be organized, knowledgeable about a wide range of subjects, and a real people person (source). You'll have to get along (and know how to read) lots of different people from many countries and cultures. You'll be interrogating them, so it's best to put them at ease while you're doing the heavy grilling.
It's probably also best that you keep your job on the down low, especially if you want to work at a place like the CIA. They warn applicants against telling anyone that they're even applying (and that includes your mother—especially if she's a blabbermouth).