Home education does not have to be structured like school; it does not have to be rigid, with 'lessons' and exams or anything of the sort. Children naturally learn through curiosity, or a drive to succeed.
You do not need to be knowledgeable, though I bet you're far more knowledgeable than you realise - after all, you have an eleven year old. :)
Check out education otherwise for some helpful advice for first-timers.
If you want to stick to a school-like system, if you want him to take GCSEs, etc. then you can buy the workbooks and let him work through them. Quizzes, answers and information can be found on websites like BBC Bitesize.
I was home educated from the age of eleven by a father who got about two O-levels. We watched schools' programmes, we went on walks and bike-rides, we read books and we talked about every subject that I was interested in. If he didn't know anything, he'd take me to the library or find me a book so I could learn more about it. I decided to take GCSEs so I studied maths in a very formal manner, but for everything else I simply read and read and read. I'm currently in my second year of university. :)
Hope that helps or reassures you a little - you can do it!