Almost always Yes.
"I am new to BJ. Is card county necessary if you plan to play and win at BJ or is there another way? I see books that claim that but...."
For some reason, Blackjack seems to attract every kook idea you can imagine. Phases of the moon, progressions, the earth is flat, have all been written about on how to beat blackjack. There are other ways of beating the game, but they all involve counting as a starting point or foundation.
"Can anyone learn to count and play..."
For the most part, if you are willing to expend a little effort learning. It is very hard at first, but with practice it is as easy as falling off a bar stool.
"or do you realy need to me very mentally sharp..."
Read posts from a lot of the people here and you will see mental sharpness is not a requirement ;>)
"good with numbers..."
If you call the ability to add 1 + 1 + 1 remember that total of 3, and a little while later subtract 1 from it, to arrive at an answer of 2, then yes, you do need a little skill with numbers.
"and be very good at staying focussed?"
Focus is a VERY good word. Like I said, counting gets easier and easier the more you do it, but focus is a very good trait to have. Also, Discipline. The ability to continue to make the proper bets at the proper time, even when you are losing, without falling into bad habits. The discipline to force yourself to practice. The discipline to walk away from bad games. Focus and Discipline will take you a long ways.
"Where do I start at? What book?"
"KO Blackjack" (which I think you can buy here), is a good book to start. Read the book, practice what it says, post questions here, you are on your way ...