PSYOPS is a branch of SOF (Special Operations Forces), like Civil Affairs, Green Berets, and the Night Stalkers. As Casey said, it is not something you can branch straight out of USMA. You can volunteer to go to selection as a 1LT. Your basic branch does not technically matter for this, but according to the recruiters I've spoken with, combat arms and MI are favorable. If selected, you will go through a 1-2 year training pipeline, incur some additional ADSOs, etc.
If you come to the Academy you will have the chance to talk with many SOF officers, both instructors and during Branch Week. DMI brings in soldiers from every branch with static displays, to include the Ranger Regiment, PSYOPS, SF, CA, and the 160th. You will get to talk to them about their experiences--many are Old Grads themselves.