I was asked this question recently. Well, besides the obvious for most people which would mean giving up a paying job,forfeiting healthcare benefits, vacation time and potentially other perks to take a risky leap… Here’s some of the immediately challenges one will likely have to face

Are you going to have business partners? If you do, choose wisely and make sure they are willing to put skin in the game.
Making sure your product or service is the best in a competitive market. You have to focus intently on truly having something that is great and people are willing to pay for.
How you are going to build a reputation when you have none? You have to get that first client which is very tough. Once you do though, they can become a great advocate for you.
Do you have the ability to persevere even when told NO by many potential clients? You’ll run into many of these NO’s as most customers are weary at first of new businesses.
People. If you are going to have employees, eventually you’ll have to learn how to manage. If you can’t lead people and inspire them, you won’t be successful.
Knowing your numbers. Once you get your company going you need to know key financial metrics, how to read a profit & loss statement, etc. Know your numbers
Obscurity. Probably one of the biggest challenges. Most people underestimate how hard it is to get your “name” out there as a company. You can have the greatest product or service but getting customers to know who you are is a major challenge, especially in a world of so much marketing “noise”. The good news though, is if you have a niche, your market is much more focused.
How are you going to fund your startup? Most entrepreneurs will have to bootstrap their venture and live frugally. My wife and I sold our home, moved into a much more inexpensive place and lived off the money for nearly a year before we took any pay.