Reliable resource for intermediate CS?

I'm 16 and I found out that there were way too much material for CS. And I'm lazy to filter them out. I tried these "Online CS Courses" but I found them too difficult(e.g. what is union? ). I know and understand the very fundamental elements of CS(e.g. tree, array, hash, bitwise logic, boolean, sorting, etc.), but how the am I going to continue CS without appropriate material? I wonder if I should buy 'The Art of Computer Programming', by Donald Knuth. Was it a good book for me?