Employees make mistakes, and even if you're not new to the management game it can be difficult to know what to do about it.
You do the best you can. When you know better, you do better. ~David Hira
If someone under your care messes up, the first thing you should do is ask yourself:
"Are they trying to do their job correctly?"
If the answer is Yes, then this is an opportunity for you to continue their training. Help them understand why what they did was a mistake, what the expected course of action from this point forward is, and why. If the answer is No, however, then this is the time for disciplinary action. If they consistently are making the same mistakes even after proper instruction, then they are actively wasting your company's time & resources.
Too often I see managers writing people up for simple mistakes that were the result of the manager's own failure to properly train their employees. Disciplining someone who is genuinely trying to do good work will only erode any enthusiasm or loyalty they feel to your company. As a manager, it's your job to tell your employees what's expected of them, give them the tools to do their job, and empower them to use those tools.
Mistakes will be made, and that's just par for the course. What you shouldn't do is punish those who are trying. Otherwise, you'll create an environment where honest hard-working people will be too scared to do anything, and that is the biggest mistake you can make.