Please tell me the way to buy Keil, I couldn't see the price and instructions. I'm in Vietnam
You're going to need to try a lot harder than that if you want to make progress with embedded systems development!
Just click on 'Products' at the top of this very page. That takes you to:
http://www.keil.com/product/
On the left-hand side, you will see a section titled 'Distributors'...
Or you could click on 'Contact' at the foot of this very page. That takes you to:
http://www.keil.com/company/contact.asp
That page also gives you a link for local Distributors, plus the Sales contact details for Keil themselves.
Just send your request to sales.intl@keil.com along with full address information.