The best advice I have heard is this: You have to start before you are ready.
Some think that you have to have everything planned out and all of the potential downfalls covered including a full blown (generally total speculation) business plan and then after all that time and effort you might be ready to begin. The chances are that under this method, you will never get it going at all.
With a clear vision of what you want to accomplish and the will to proceed, you will learn what you need to know and you will find that your mental plans are being continually improved as you move forward. What you will end up with is far greater than if you tried to cover all the bases ahead of time. I don't think it is possible to cover all the bases ahead of time. Part of the growing process is refining your vision and goals. That comes from experience, not planning.
If you have a good idea and you feel strongly about it and are willing to commit to it, rather than just try it out, get going! The answers you need will come as you move forward. It is absolute fact.