Arduino + USB

Hello There, i'm want to get signal from a USB keyboard.
I have a USB entry, I already know the data +/- gnd and vcc.
But i can't see the signal, so, can you tell me how to make a Homemade USB host ?
PS : I can't buy a usb host, need to be cheap because it is for my school.
and sorry for my bad english ;D

Arduino as USB Host requires an Arduino Due.

http://playground.arduino.cc/Main/InterfacingWithHardware#keyb
This should give you a hint that USB is not that easy (requires extra hardware)

If you find an old PS/2 keyboard, things are different. Covered in the same playgound chapter.

I do have an old ps2 keyboard, but i need to make with a USB.
And i don't have an arduino Due, i only have arduino uno.

And it is possible, read data from a usb and translate in decimal or characters?
so if that is possible, how a USB send and read data? is in binary?

This tells you how to use a ps2 keyboard with an arduino
http://playground.arduino.cc/Main/PS2Keyboard

To make this look like a keyboard to a computer using USB you need an Arduino Leonardo or a Micro.