In my experience, no entrepreneur is fully prepared for the journey they are embarking upon when they start a new business endeavor. You simply can't have everything you need before you begin this journey. Being an entrepreneur is about being resourceful, or as Paul Graham puts it, being "relentlessly resourceful". You'll need to solve problems as you go and you can't anticipate every hurdle you're going to have to overcome. Ask people around you if they would describe you as resourceful. Ask yourself this question. It is the one skill that every entrepreneur needs to have.
That said, I think you can try to stack the deck in your favor before you begin. Money, knowledge, a head start on your product or service idea before you go full time, the right partners, and the support of your significant other are all things that will greatly benefit you.
In the end, you're going to have to be willing to do whatever it takes to make your business endeavor a success. You're also going to have to be willing to work hard and live an unorthodox lifestyle while you are getting things started. If you're not looking for this kind of life and you don't want to give up a lifestyle that you've become accustomed to or if you don't have the confidence that you'll be able to figure it out as you go, then you may not be ready yet.
I've always thought that you never really know what you are capable of until you are put in a position where there is only one way forward and failure is not an option. In this respect, sometimes I think it's worth a try if starting a new business endeavor is something you've thought about for a long time. Don't be afraid to put yourself out there to see what you can do.
I know my experience of starting a business was one in which I didn't have many things going for me, but I still managed to find a way to make it work and find success.
Work hard. Don't give up. Strive to be the best. Do whatever it takes. You can do it!