I did this, and had a successful software startup that was aquired. This is not as easy as it seems. I have two boys, and I'm now divorced. The business ruined a marriage, friendships, though I made new friends, and now I'm sitting on a pile of cash. I'm not happier than when I was working for Microsoft, or Cisco Systems.
First, you need to have your spouse onboard. When I mean onboard, I mean she needs to understand that most of homelife is going to be shifted soley on her. I cannot stress this enough. This means kids birthdays, soccer practice, ..., you know the drill if you're a parent. There is almost a 100% chance you will end up divorced, during the process, or after at somepoint. Doing this is going to change you, and change your spouse. I cannot describe the amount of stress you're about to endure. If you're going home at 5pm, you're doing it wrong.
You need to define goals for your personal life, and for your business. They need to be detailed, and aligned. If you miss a major milestone, things just got that much harder.
Next, you need to have a product (at least a prototype). "looking for product-market-fit" is not how you go into this. You should have identified a problem others are having, and know the solution before you even start.
You need a business plan, it doesn't need to be formal. You need to know your burn rate, and what it will cost to get you to the next steps, which is to find an angel or other investor, or revenue stream that funds you, and then to productization, and whatever your exit strategy is going to be. Remember above goals.
I contracted my engineers out to help fund our main product line, after I had my first contract in place, and then was able to bring on investors. Find people with investments in oil & gas. They have tons of cash, and they don't mind losing it. Having the revenue stream helps potential investors. You learn why.
Depending on your product/service, you're going to need a customer. This is the really hard part. You need to find someone that needs your product, that is willing to go through the trials and tribulations with you.
Hire an attorney, or at least find one that will work with you. If your product interrupts the market in anyway, be prepared for lawsuits. I had two lawsuits that I won both. It cost me a portion of the company. I had no way to pay them.
Do not give equity out. Equity is worthless until it is not. Beware of the wolves, and sharks. Wolves want to be part of your pack, and feed on your winnings from the inside. Sharks are looking for a quick buck. They are easier to spot. Only work with people that can contribute.
The single best advice I received was from the president of Challenger Capital. He told me this, "Go for the throat, because if you don't, they will." That was the only piece of advise he was able to give me, and it turned out to be the best advise I could have received.
Your first startup will be the hardest.
I started my first company with $2400 dollars in my checking account, a pregnant wife, a mortgage, and very little debt. Six months after the aquisition, my wife filed for divorce, and then tried to take it all.
If I am really honest, looking back; I don't know if I would do it again.