Your home cellular service provider can unlock your phone for you. You can find information about your carrier's unlock policies by visiting their website or calling their support line. You can also unlock your cell phone with UnlockBase. UnlockBase has provided phone unlocking services for over 10 years for a wide range of popular devices.
Unlocking refers to removing the network restrictions so that the phone can be used with a SIM card from a different network. Phone unlocking is not the same thing as “jailbreaking” or "rooting". Unlocking simply allows for you to use a different mobile network with your phone. Generally, jailbreaking and rooting refers to changing the restrictions, allowing you to download non-standard or unofficial software.
Also keep in mind that even if your carrier unlocks your phone they may have actually only unlocked it locally. Mostly in the US, some carriers will unlock your phone locally which only allows you to use another SIM card from that country but not an international SIM card. Make sure to clarify with your carrier that your phone has been unlocked internationally.