What do you need?
A Raspberry Pi, model B.
A boot SD card for the Raspberry Pi.
A USB WiFi device that supports "Access Point" mode.
An Ethernet cable to connect to the local network.
How does it work?
The Raspberry Pi is configured as a WiFi Hotspot, just like you would see in an internet cafe. It allows you to connect to the internet over WiFi using the Raspberry Pi as the bridge to the internet. The basic steps are