Starting a company is difficult--just ask anyone who's ever done it. But what's the single biggest challenge for a brand-new company owner? Different entrepreneurs have different answers, and different stories to tell. To get a full picture, the personal finance site GOBankingRates collected comments from 24 of the world's best-known entrepreneurs and media icons. Here are a few of their answers--you can see all of them here.

1. Mark Zuckerberg: Not knowing enough.

"When I was getting started, I didn't want to build a company and I didn't know anything about building companies," the Facebook founder explained in a Q&A session. The thing that got him through, he adds, is help from the people around him.

Lesson: Don't expect to know everything yourself. It's OK to ask for help, and to admit what you don't know.

2. Mark Cuban: Hearing 'No.'

Don't be an entrepreneur if you can't handle rejection. One of the toughest aspects of entrepreneurship is getting negative answers time and again, according to the Shark Tank personality and Dallas Mavericks owner. No, you can't have funding. No, your business model doesn't make sense. No, we don't want to buy your product. But, says Cuban, "Every no gets you closer to a yes. It's a numbers game."

Lesson: The more nos you hear the better. It means you're out there pitching. So thank the person who said no and go on to someone else. Eventually, you will get a yes.

3. Tory Burch: Projecting confidence.

The entrepreneur and fashion icon admitted in a Charlie Rose interview that one of her biggest challenges was "gaining confidence to really believe in myself." When her first interview was published, a friend pointed out that she'd avoided the word "ambition." Burch realized her friend was absolutely right.

Lesson: It takes confidence to run a business, just as it does to wear stylish clothes well. If you're feeling tentative, look for ways to boost your own confidence. You need your employees and customers to believe in you, so start by believing in yourself. If you can't, try faking it till you make it.

4. Sergey Brin: Learning to scale.

It's an old story: Serving a few customers is easy, but when your business starts to grow, everything gets more difficult. That's a lesson Google's co-founder had to learn as the company grew from 10,000 daily searches when it was founded at Stanford to over 50 million today. "That scaling of an infrastructure, that is pretty challenging," he said in an interview.

Lesson: Think big from the beginning and be ready to adapt as you grow.

5. Travis Kalanick: Controlling your own passion.

It's a delicate balance. Entrepreneurs need passion to succeed, but too much passion can threaten success just as much as too little. "I'm like fire and brimstone sometimes," the Uber co-founder said in a magazine interview, describing his entrepreneurial passion. But, he continued, at times he gets "too into the weeds, and too into the debate, because I'm so passionate about it."

Lesson: It can be hard for passionate entrepreneurs to get an objective perspective on their products, industries, or companies. You need advisers you trust who can tell you when to dial it back.

6. Dr. Dre: Hiring the right team.

Your team has to completely share your vision, says the rapper and Beats Electronics founder. "It takes a while to get the right people around you; it takes a long time," he said. It looks like he's finally gotten there, he added, and based on his success, that would seem to be true.

Lesson: Hiring the right team requires patience. Don't rush it! Take the time to find people who are as passionate as you are about your company or product, and who fit well with your working style and the culture you want. You'll be glad you did.

7. Jack Dorsey: Doing things you're not good at.

The Square founder and Twitter co-founder struggled with some business tasks, such as bookkeeping, which he says he learned by trial and error. "I had no idea what I was doing!" he said in a recent speech. "I learned as I went, working every shift, every day."

Lesson: Hang in there! You won't know how to do everything you need to do right from the start. Learn from your mistakes and then keep on trying.

8. Oprah Winfrey: People wanting you to fail.

The daytime TV talk-show star said founding her own network, OWN, brought nay-sayers out of the woodwork. People, she said, were "lying in wait for you to fail, or to make a mistake."

Lesson: She pushed on and so can you. Yes there will be people--envious people, mostly--who really don't want to see you succeed. Which will make it much more satisfying when you do.