Study hard. Chances are your grade-point average will need to be at least in the top 10 percent of your graduating class.
Learn a foreign language and a fair amount of math, as these are important selection criteria regardless of chapter.
Find out the contact information for the Phi Beta Kappa chapter at your college.
Contact the chapter office to find out selection criteria, but don't bother applying - the initiative for nominations rests with local chapters.
Do at least 3/4 of your college work in the liberal arts or sciences, as these are the only fields covered by Phi Beta Kappa.