As far as I concerned, there are plenty of such websites and it's pretty difficult to determine which one can be the best for gaining knowledge in trading. You shouldn't forget that knowledge also comes with experience and skills. If you will start trading on a real account, then probably you will be able to gain knowledge and experience simultaneously. I believe that the best website for gaining knowledge is something like investopedia, because all the basic notions are described there. Moreover, you can read additional information about traing in case you want to dive into it.