The CIA seal.

Does that James Bond movie marathon you watched on TV have you dreaming about being a secret agent? If you’re a U.S. citizen, you may be wondering how you can get into the Central Intelligence Agency (CIA), the nation’s top career choice for aspiring spies. Before you get your heart set on joining the Agency, however, there are a couple things you should know. First, the Directorate of Operations, or “clandestine service” (the branch of the CIA that includes spies) makes up only a small percentage of the CIA’s total employment; most CIA employees hold fairly mundane office jobs. Second, the selection process for any job within the Agency is rigorous, and even if your experience and education qualifies you for a position, you can be turned down for many other reasons. Still want to give it a go? Here’s how.

[edit] Steps

Obtain U.S. citizenship. Only U.S. citizens may enter the CIA. If you are not currently a citizen, you’ll need to become one before you can apply.
Stay squeaky clean. The CIA requires security clearances for all positions, and you’ll need to be able to pass a very thorough background check to get this clearance. It’s not made public exactly what the background checks look for, but as a general rule you should be a model citizen starting quite young. Don’t get involved with criminal activities; be responsible, ethical and dependable at work; maintain good credit; avoid gambling; be trustworthy; and be faithful to your spouse and to the United States of America. You don’t need to be perfect, but the CIA places very high importance on personal integrity, sound judgment, and loyalty to your country. Your parents and friends must also be as squeaky clean as possible.
Avoid illegal drugs. The CIA states that in order to be eligible for employment, you cannot have used illegal drugs within 12 months of your application or background check process. Illegal drug use at any time in your past can hurt your chances, however, so it’s best to stay away from them altogether. You also shouldn’t abuse legal drugs, such as alcohol.
Go to college and get good grades. You have to make sure that you meet the educational requirements for a position. Almost all CIA jobs require at least a bachelor’s degree, and advanced degrees are helpful for most positions and required for many. You should have at least a 3.0 GPA on a 4.0 scale. There is no specific major that you should take, but candidates with emphases in international relations, area studies, economics, math, and science (either behavioral, physical, or computer) are usually in high demand. Take serious, rigorous courses, and perform well in them.
Learn a foreign language or two. Fluency in one or more foreign languages is important for many CIA jobs—especially those in the clandestine service. Particularly valued are Middle Eastern and Asian languages such as Arabic, Mandarin, and Farsi that are spoken in many of the world’s political and military “hot spots.” A course or two in college won’t cut it; you should develop language proficiency as close as possible to that of a native speaker.
Search the current employment opportunities at the CIA’s website. The job listings are updated regularly, so if you don’t see something you’re interested in, check back later.
Apply for a specific position. Once you find a position you’re interested in, carefully examine the job description to make sure you meet the minimum requirements. Submit your resume and application via the CIA’s online application.
Wait. As a general rule, if the CIA is interested in your application they will contact you within 45 days. Don’t bother writing or calling to check on the status of your application. If you get a conditional offer (all initial offers are conditional), you’ve still got a long way to go before you can be hired.
Undergo a medical exam and psychological exam. The medical exam is designed to ensure you are in adequate physical condition to meet the needs of the job, and it is also used to test for drug use. The psychological exam assesses your intelligence, judgment, and mental stability.
Wait for your background check to be completed. The background check is extremely thorough and often lengthy (it sometimes takes as much as two years). According to the CIA website: “The investigation addresses comprehensively one's loyalty to the United States, strength of character, trustworthiness, honesty, reliability, discretion, and soundness of judgment. In addition, it examines one's freedom from conflicting allegiances, potential for coercion, and willingness and ability to abide by regulations governing the use, handling and protection of sensitive information.”
Pass a polygraph examination. Before you can be hired (and periodically throughout your employment with the agency) you will need to pass a polygraph (lie-detector) test. While polygraphy isn’t an exact science, the CIA polygraphers are among the most thorough in the business, and they’ll generally err on the side of caution if they have any reason to believe you’re lying.
Accept or decline your job offer. If you make it through the selection process, you’re lucky: only about 17 percent of candidates presented with conditional offers pass the background check and exams. Now you can accept your job and get ready for training.
Participate in job training. For some positions, particularly those in the clandestine service, you may have a probationary training period during which you must successfully complete training for your specific position. You may have to relocate during your training period (about six months), and the Agency will usually not pay for relocation of your family during this time. You will not be an official case officer until you pass the rigorous training, which, depending on the position, can be very difficult.

[edit] Tips

The CIA usually won't hire anyone who is over 40 years of age.
Join the military. Although the CIA offers no hiring preference for veterans, candidates with relevant prior military service, especially in military intelligence, have a leg up on the competition. You’ll still need a college degree, however.
The CIA has a number of programs available for college students, including internships for both undergraduate and graduate students. The selection criteria is quite rigorous for these, and completion of a program certainly doesn’t guarantee a future job, but if you perform well during the internship you’ll have a good chance at a job offer.
The CIA frequently recruits at top colleges and universities in an effort to lure qualified candidates before they accept a position in the private sector. Be sure to attend career and job fairs at your school.
Make sure to follow the application deadlines and instructions to the letter. If you miss one thing, your application will be rejected.
You will be required to keep things confidential. People skills are a plus for operations officers (spies), but if you have trouble keeping secrets, this is not the job for you.
You will be placed in a position you are qualified for, which may not be the one you initially applied for.
Be prepared to relocate and be away from your family, depending on your station.
The salary will depend on your grade. There are regular salary increases over a period of time. The starting salary is around 40k. However, the perks are really great!
Some benefits are: Paid Time Off, Federal Health and Life Insurance, Retirement, Education and Training, Health Services, Child Care Centers, and Credit Union.
Some languages that are currently in high need include: Arabic, Central Eurasia, Chinese, Dari Pashtu, Farsi/Persian, Greek, Indonesian, Japanese, Korean, Russian, SerboCroatian, and Turkish.

[edit] Warnings

Do not lie on your application or at any time during the hiring process. The background checks are extraordinarily rigorous, and should they discover a lie you will be disqualified. Be aware that the different departments in the intelligence community communicate with each other. Being disqualified/fired from one will make it harder to get a job with any other agency.
Be prepared to handle the emotional stress. Training, as mentioned, is very intense (and at times, overwhelming).
Be aware that salaries in the CIA are usually lower (sometime a lot lower) than those offered in the private sector for positions requiring similar education and experience.