I will respond to this in a bottom up approach. Typically the least level of education to really begin being an effective provider of care in a psych unit or hospital is at the masters level either as an LPC (Counselor) or as an LMSW (Social Worker). Then beyond that you have the PhD or PsyD in clinicial/counseling psychology or MD as a psychiatrist. The requirements for each vary depending on the state you want to practice in and each group has a unique prespective in providing care to there patients/clients. I really recommended that you speak to an advisor at school or a career counselor that can help you choose the path that best meets your goals and strengths.
As a quickish break down
LPC - Lisenced Professional Counselors require a masters in counseling typically at least 60 graduate hours of education and then a 3000 hour supervised internship.
LMSW/LCSW - Lisenced Clinical Social Worker requires a masters, I do not know how long but I would assume equivalent to a counseling masters at least in the 48 graduate hour to 60 graduate hours of education and then a 3000 hour supervised internship
Phd/PsyD - Is a Doctorate in psychology. Someone else will have to fill in some more details for me here, but I know it requires at least a 1 year supervised internship
MD - Psychiatrist requires you go to medical school and then complete a 4 year residency in psychiatry before practicing independently.
Now remember each has strengths and brings something unique to the table when helping patients/clients so do not allow career bashing to sway any decisions you make, when you choosing a field.
Jeff