Believe it or not, the BEST way is...... GET GOOD GRADES in School, DON'T tell anyone that you want to work for the CIA, learn at least one language (Cantonese Chinese would be a good one), keep in good physical shape.
Then, about the time you finish your first year of college, Write a letter to the CIA Office in your district (yes, they have one), asking how to go about employment opportunities. Anything further detailed than this you would ask IN PERSON and make damned sure that you weren't running your mouth about it. I have also seen were people were invited to be interviewed after several years of Military Service when they showed interest. (That is when I was given a chance to interview.)