You need to have a live/outlook account and Windows Phone Developer Registration tool for unlocking the device to install the apps on the device. This tool is installed along with your Windows Phone SDK.
To install the app you need to unlock the device first, for this you need to connect your Windows Phone via USB to system and start the Registration tool. The tool detects the phone and asks you to login into your Microsoft account(live/outlook) , after this step the phone is successfully unlocked.
After unlocking the phone you can use visual studio or Windows Phone Power Tools or Application Deployment tool to install applications on the device.
Previously we need to have a developer account for installation of apps on devices but now a Microsoft Account(live/outlook) is enough for doing so.