The first critical piece of advice that I can provide anyone is that you must be passionate about what you are doing before you even consider launching a new business. Why? Because you will work hard, possibly harder than you have ever worked for a previous or existing employer and you may not earn money right away. Always consider your financial situation and I highly recommend having six months of living expenses in the bank before launching a new venture if you plan to quit your job.
Next evaluate your own skills and knowledge, and it is imperative that you be brutally honest with yourself. Identify what you are an expert in, identify what you have some knowledge about but clearly do not consider yourself to be an expert in, and identify what you know nothing about. When you understand your strengths as well as your weaknesses you gain perspective and should consult experts when you know nothing about a topic.
Finally take the time to map out your goals and game plan before launching your new business. Always look at other entrepreneurs and successful businesses related to your business. How are they similar to yours, what can you learn from their business, what processes can you use from their business, how will yours be different? Always research what has been done in your industry before you and consider these words from Sir Isaac Newton "If I have seen further it is by standing on the shoulders of giants".