I'm guessing you're from Asia just by looking at your name. Find out the model of your Galaxy S3 by into Settings > About Phone. There's should be a model number starting with GT or SGH. Asian versions start with SGH usually. European and US models start with GT.
Once you've found your model number just google "Root Samsung Galaxy S3 [model number here]" You'll find tons of results. I'm not going to link you myself because I don't know your model number. Make sure to use the correct one for your phone, rooting the wrong version can seriously fuck up your device.