Three main aspects to consider when starting a business are your personal level of commitment (starting a business is hard work), your business plan (do you have a viable idea that is positioned for success?) and the legal issues (starting it right leads to fewer problems down the road).
Your Commitment - Running a company is completely different from having a job. Skills that made you a valuable employee may not be relevant in your new role, and you're certain to need many additional skills you've never needed before. Think about your available time, drive and money before you turn in your resignation. The beauty of starting your own business is that you can set the hours and work part- or full-time. But if you aren't starting with clients already in hand, you'll need extra time to find some.
In terms of capital, savings, venture capital, banks, angel investors, government funds and credit cards are the main sources to consider. How will you cover your expenses until your revenues grow? Another important issue is your "why." Why do you want to work for yourself? Some people prefer to be self-employed while others perform better as an employee. Knowing yourself and being clear on exactly what you want to accomplish can play a major role in keeping you focused and motivated when you hit hurdles.
Your Business Plan - Taking the time to write up a business plan allows you to see the big picture. Somehow when it's all down in black and white you notice areas that need further clarification or maybe even an entirely new strategy. You'll also need a written plan if you're going to seek outside funding sources. The Small Business Administration has a great free tool. The organization SCORE is also a good resource, with many local offices nationwide. Your plan should include a description of your product or service, your competition, operating procedures, marketing, personnel and financial projections.
Legal Issues - There are different legal structures for businesses, and you'll need to understand them in order to decide which best suits your purpose. The four main business structures are sole proprietorship, partnership, corporation or limited liability company. You will want to understand the pros and cons of each. In terms of staffing, be aware that the IRS has received new funding to find contractors who are misclassified employees. The IRS has also recently changed the rules for unpaid internships. From now on, if an intern's work is benefitting your business you are required to pay. If you're going to use independent contractors and interns to help get your business going, learn the specific rules to avoid back taxes or penalties.