When I interview job candidates, one of the questions I almost always ask them is: "If I was going to start doing your job tomorrow, tell me something that you know that would make me better at the job that you didn't know before you started?" You can learn a lot from the answers you get. With that in mind, 2014 marks the 15th year that I've been a working "professional". During that time I've worked with start-ups in e-commerce, real estate, finance, biotech and healthcare. I've been in some fast-paced and really competitive environments. And I've made a lot of mistakes and learned a lot of stuff that I didn't know before I started.

So I thought I'd put together a list of 15 key things that I've learned (one for each year) that have helped make me successful. Specifically, these are things that I didn't know or appreciate before I got into the real world. Here we go:

1. It's a grind. Work is hard and painful and complicated. If it's not a grind then you're probably not trying hard enough. Get comfortable being uncomfortable.

2. Be candid. Try to facilitate a work environment where if someone is doing well you tell them instantly and if someone is not doing well you tell them instantly. Get used to being honest and upfront about what's working and what's not. This is only hard when your culture isn't used to it. Force people to get used to being candid.

3. Assume that most people are lazy and incompetent and what you want from them isn't important to them. This definitely isn't always true, but you're better off assuming it is.

4. There's no such thing as sales. There are just two parties trying to do something good for themselves and their families. Everything is driven by self interest. Never defer to a buyer. They're not doing you a favor, they're getting something and you're getting something. You are equals. Act like it.

5. Cannibalize yourself. Too long to describe here. I wrote a post on this a while back. But in short, put yourself out of business before someone else does.

6. Always do the the right thing. Don't take credit for other people's work. Publicly recognize your peers that are doing good things. Share your ideas and insights with other people. Don't go over your boss's head. Help people. Don't one-up people. Do the right thing. Don't think about this from an ethical perspective. That might make it too blurry. Think of it practically. I'm telling you with 100 percent certainty, it might not feel like it at the time, but I promise you that doing the right thing is better for you in the long term.

7. The worst trait in a colleague or a boss is insecurity. Avoid insecure people. And avoid insecure managers like they have a contagious disease.

8. Manage yourself harder than your manager manages you. Don't even make it close. If your performance is being actively managed by your manager you are losing. Get in front of it. Innovate on how you should be measured and developed and managed. Never fall behind on this.

9. Try to find the trifecta job. Something that you're good at, something that someone will pay for, and something that you love.

10. Firing someone is almost always the best thing for the person being fired. I've worked in a lot of cut-throat environments and I've seen a lot of people get fired and I've fired a lot of people. Not once can I point to a time where it wasn't the best thing for the person and the company. Both sides always wind up in a better place. And be respectful to people that get fired. Someone that is awful at their job could easily be a top performer somewhere else and someone that is awesome at their job could easily be a bottom performer somewhere else. It's all about fit.

11. Credentials are meaningless. I've worked right alongside several Harvard Business School grads and several software engineers from Apple. There's absolutely no correlation between success at work and these credentials.

12. Hiring good people is really difficult. The traits I look for are grit, adaptability, curiosity and humility. These things are almost impossible to measure in a traditional interview.

13. Admit when you're wrong. If you're the kind of person that can't admit when you're wrong, please stop being that kind of person. Being wrong and admitting it 1,000 times is way, way better than being wrong one time and not admitting it. Embrace being wrong.

14. Be a lynchpin. Seth Godin has a book on this that you should read. But the point is that you should run as fast as you can to be a completely critical piece of your organization. If you're not that, then try harder or move on to somewhere where you can be.

15. Always think in terms of metrics. Whenever you think about an initiative or a new role or a team structure, think of what metrics it will impact. If what you do every day doesn't impact your company's key metrics then you're not a lynchpin.