How can you get into the CIA?

The first think to do is go to www.cia.gov web page (see below in links), then click the link careers. Then, choose a job and fill the aplication. The CIA doesn't accept resumes; You need to fill their own resume format. If you are compatible, they will call you and conduct a phone interview with you. If you able to pass it they will send you the application package.
Interestingly, in Jan. of 2009, the CIA opened up the greatest known number of now job positions ever.  

What to study:
Start by studying the subjects recommended by the CIA, these are: science, engineering, languages, analytics and support Services. Decide on the subject you enjoy most and chose a 4 year bachelors degree, as this is required. It is important that you are fluent in more than one language. One of your languages will most likely be English as you must be a US Citizen.
Only then will they consider you for any position.