ano you guys must get this alot but could somebody plz help me unlock my iphone 3gs 3.1.3

If you could give me in a detailed view of what to do that would be very helpful. ive never done this before so be gental.

my fone is a uk orange and am now in austrailia and want to use my fone over here on there network Optus.

ave seen a few videos on youtube but didnt really understand it. ino u have to 'jailbreak' it first how do i do that ? then i have to use a app called Cydia ? where do i get that app from ?

any help would be very much apreciated