Some companies have moonlighting clauses and some companies allow you to work on your own side business. It just depends on the policy.

You might be totally prohibited from starting your own company. You might have signed a contract stating you employer owns all the work you do even outside of work hours. You might be allowed to create your own company as long as it's not competing with your employer or they might require you to disclose it to them.

Point being, look at your contracts. If you have an employer that would be a problem, quit that job. You're going to need to quit at least one of those jobs to have time to start your own company.

As an aside, don't work 80 hours a week forever. It's not healthy. Don't forget to enjoy your life.