how to "catch" free wifi signal and use it on my p.c????

hi i have a desktop computer. i get free wifi signal in the balcony of my room. my friends who have laptop use the wifi without any problem.my p.c is inside the room and the wifi signal does not come inside the room i have checked this by using friends laptop,the signal comes only in the balcony. and i cannot move my p.c in the balcony.
how do i catch the wifi signal i.e connect my p.c to the wifi..
what kind of hardware do i need.
as far as i think i need some device that would catch wifi signal(like the wifi thing which is inbuilt in all laptops nowadays), which i will place in the balcony then i would need some wire to connect the device to my p.c.
pls tell me what hardware setup i need and it would be great if u cud give idea about the price of the harware.
thanks.