If the possum is longer than a dollar it's actually not a baby (though they look it) and capable of taking care of itself. If it's smaller than that call A.W.A.R.E. and they'll help!
You can call A.W.A.R.E. and leave a message. They will call you back and tell you what to do.
How long has it been? Mom may be hiding but may come back. I guess you shouldn't leave the baby alone for too long but you may need to research how long the baby can survive without the mom before getting involved.
Thanks! Your answer is awaiting moderation.