One of the many things that is so important in life is to learn from our mistakes, but in order to learn from mistakes, we first have to be willing to make them. Mistakes aren't made and lessons aren't learned by those timid souls who sit idly by and wait to be told exactly what to do. When you start in a job, you may not quite know enough to do a lot of things on your own. If you don't, be proactive in asking your boss what you can help with next. Don't sit and wait for her to come to you. If you learn quickly, hopefully you have a boss who will encourage you to make decisions and will give you the authority to make them. Know this though, eventually, you are going to make a decision that is a mistake.
When you make your first major mistake, I hope you have a boss who works through it with you, who explains why it was a mistake and what might have been a better option. Enlightened bosses will know that you're going to eventually make a mistake. They hope they can catch you before you make it, but that isn't always possible.
An important piece after you make a mistake is how you communicate it. When you do make a mistake, own up to it. Don't blame someone else if you know that you screwed-up. None of us wants to think that we're the ones at fault, but when we really think about it, we might be. It wasn't done on purpose, but it was our fault. Our knee-jerk reaction is to blame someone/something else. I've worked in organizations where the first thing out of people's mouths when they screw up is "such and such was supposed to ________, that's why it happened." They also love to tell everyone else that same thing. Please don't do that. Be an adult. Don't start telling others that it was someone else's fault. Be willing to say, "It was my fault. I screwed up. It's a lesson I've learned and it won't happen again." Seriously think about what you learned from the mistake--you should have followed up with someone or you should have gotten the price quote in writing or whatever.
The next key is not to let it happen again. Making mistakes isn't career killing, but making the same mistakes over and over is. You don't want to become known as the person who always screws up. You also don't want to be known as the person who doesn't take responsibility for her actions.