1. Get a lawyer. Advice from user forums is *not* an acceptable substitute. They can help you avoid stepping on toes of the big boys like Microsoft and AutoDesk.
2. Check the tax laws. The lawyer may be able to help with this.
3. Triple check for ethics violations. They may not seem like much, but if this "takes off" they will grow large teeth and take a big chunk of meat out of you. Examples of this are developing code on a companies computer and then turning around and selling it to the competition, even if its several years later.
4. Research the competition. See what they are doing right, doing wrong, and how much they want for doing it either way.
5. Get a lawyer. Yes, I know its the same as (1). Its that important, its worth mentioning again.
6. If you got your code from somewhere else, include credits for it in the documentation, no matter how small the contribution.
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If you are going to fly by the seat of your pants, expect friction burns.
"I don't know" is the beginning of knowledge, not the end.