Answer: Your manager obviously did not handle this well. If he was disappointed in your performance, he should have talked to you when the problems arose instead of blindsiding you with a written warning.
At this point, however, you need to consider two key questions. First, do you understand what he expects you to do differently? And second, do you have the ability to make those changes?
If you were given only vague directives, like “show more initiative” or “communicate better,” then you need to request more clarity. Ask your boss to help you make a list of specific goals to be accomplished or behaviors that need to change.
If you are capable of making these changes, do so immediately. If you lack critical skills or knowledge, then ask for some training. But if the expectations seem impossible to meet, that may be a sign that you are simply in the wrong job.
Hitting this roadblock in your first professional position might understandably make you feel shaken and discouraged. But don’t lose your self-confidence. Instead, try to focus on what this experience can teach you about yourself, your manager, or the kind of work that you prefer.
For suggestions on recovering from a negative appraisal, see How to Respond to a Bad Performance Review.