I started a software company in 1993 and sold its intellectual property in 1996 for 7 figures. I currently own and part-time operate another software company. I am a full time university professor, and I still have consulting clients.
My recipe:
Step 1) Find out what some customer who has deep pockets wants. Believe it or not, they will often tell you what they want. It helps to already have a business relationship with the customer. I don't recommend cold calling. Step 2) Create whatever it is they want. You will need to be able to eat and shelter yourself while you do this. If you can't do it alone, you may need to pay employees or contractors to help you. Step 3) Hire a lawyer and an accountant. They can be the same person. Step 4) With your lawyer and accountant, determine how best to organize your company. I am in the USA and ended up with an S Corp. Other options range from sole proprietorship to partnerships to C corp. Step 5) Sell your software or licenses to your software to the customer.
Depending on how much investment it will take to complete step 2, you may need to reorder step 2 after step 4.
What not to do: 1) Write software hoping to find a customer later. You are taking a big chance. If you make a mass market product, how will you make your potential customers aware that your product exists? How will you collect money from them? Modern App stores help with this, but in my opinion, it is much less risky to build exactly what you already know someone who can pay a lot of money wants. 2) Look for venture capital. These guys know that a good idea is worthless without great execution. What they care about is technical and business capability to make the software. Unless you have a long and glorious track record of success already, the venture capital company's first move is to kick you out so they can bring in "real" "trusted" talent/management. 3) Ask banks for a loan. Unless you have a long and glorious track record of success already, they are not going to lend any money to you.
In my case, I was 2 years out of college with no debt and a wife who earned enough to support both of us. My wife and I made a deal: I will quit my software job and try creating a business for 2 years. If it isn't going well at the end of 2 years, I will get another job as a software engineer, and we will start a family.
My former employer asked me to do some consulting after I quit. I earned more money consulting in the first 6 months than my wife's annual salary. She quit her job early and we started a family. While consulting with various companies, some of them explained in detail what they wanted. Over a couple of years interspersed with consulting, I built what they wanted. I invited the customer's executives to come see what I built, negotiated for months, and eventually agreed to be paid mid 7 figures in installments over several years in exchange for transferring my software intellectual property and supporting the customer for several years.
I became a millionaire at age 27. The company grew to 11 employees including myself and my wife. Our customer used our software for competitive advantage, and as a side effect, our software was featured on the cover of every industry magazine and in several trade shows. At one of the trade shows, our customer mentioned my company by name to the press while being interviewed about our software. There was no shortage of new customers for a few years after that. Business dried up in 2002 in part because of the 9/11 2001 attack. I shut down the company after incurring about a million dollars of operating costs without any return. All of my employees landed on their feet.