Samsung Galaxy S3 — Changing ownership Galaxy S3 locked to Telstra and not reformatted (given to me)

Yesterday I was given a Galaxy S3 phone which has almost never been used. I think it is locked to Telstra and stiil has some of the previous owners info which I want to erase. I haven't been able to connect to my broardband wifi. I notice a post re unlocking phones, I will go there and try that, but I think I'll need an internet connection first.