how to shave a cat's hair?

see this website
http://www.wikihow.com/Shave-a-Cat

I would highly recomend taking her to a groomer to have this done. You don't want to traumatize her or more important, you don't want to hurt her.

and while you are there get a good brush and brush her a few times a week so you don't have to have this happen again.

So long as your cat is a "cool" cat you can use human hair clippers.
You may need to get the vet to do this as, if your cat is NOT a "cool" cat, you could end up in ER!!!
Your cat may need sedating for this. It can really pull an already uncomfortable area.

if you want lots off then take it to a grooming palour, it probably wont let you do it your self, but if you just want to chop of matts then you can use hairdressing scisors.

n.b. but be very careful and only if your cat is relaxed about things like that, you dont want it to squirm and you acidentally cut it, if it tries to run away stop and try again the next day, or get it done professionally

Shave a cat? Is it possible on earth? I have never heard about it.

I personally feel cats are cute with their hairs. Dont cut them off - rather you can use a comb.