If it is due to being locked to your carrier/service provider that you had before, you need to contact them to have them unlock it. No one else can.
However, it is possible that the phone is a totally different type of network than you are trying to use it with now. In the US carriers are either CDMA or GSM. So if say the phone was GSM here, but you are trying to use it on a CDMA network elsewhere, it won't work.
So you need to also find out what network your old carrier/service provider used and make sure the new SIM actually works on the same network.