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It depends on what state you are in and exactly what kind of psychology professor you'd want to be. If it's for a college you'll need at least a Master's degree in psychology to teach that subject and if you want to be a professor in middle or high school you'll need a bachelor's degree.
In Florida (where I live) once you have your degree you also need to send in your transcripts and take an educators test in the subject of Social Science. To find out what the exact qualifications are for middle/high school professors in your state I recommend looking up the website for the board of education that you current reside in. To find out the individual requirements of the college you'd like to teach at, I'd go to the website of that college and begin searching the site or making inquiries.
I hope this helped.