Becoming a company is simply a matter of registering an enterprise with your state and with the IRS. It is a relatively simple matter when you are ready.
Becoming ready is where the business plan comes in .
Don’t leave your job without a plan. Develop it in parallel with your current work. Insure that you are not moving into a field where your current employer has you signed to a non-compete agreement, a non-disclosure agreement or some other piece of paper you signed with your employment that would keep you out the business.
Then - follow your value system. Examine your talents, your attributes and your affinities and develop an idea for a business that you would enjoy.
Evolve your plan as you continue working for your current employer.
Give your start up idea the following test:
Do you have a product or service niche in mind?
Do you believe you have a market for 1 above and the means to reach it?
Are you willing to develop a business plan to validate 1 and 2 above before you launch?
If the answer to the above questions is yes than take the actions indicated, observe the results, then make an informed decision on whether or not to proceed. For further guidance on this type of planning, please see the answers to the following question here at the Micro Mentor Q&A Feature: https://www.micromentor.org/question/4871
When you launch if your plan risk analysis indicates high early challenges, phase into them gradually while retaining your present employment. As the business matures and your risks are mitigated, cut over to full time and resign your old position when you are comfortable doing so.