I have developed a set of software myself. I have no patent or company. Can I sell it? Do I need to pay taxes after selling it? Do I need to sign a contract? How to do invoicing?

Has the software been tested? How is the stability and security? Are there any sales channels? Generally speaking, the source code is relatively difficult to crack as long as you don't leak it yourself. So there is no need to apply for a patent unless your customers are also software vendors. If the software is good, you can donate it by leaving your account number in the software description and asking users to donate. Although the money will not be much, what if it becomes popular?