There are several internal root managers in the Google Store but I always use KingoRoot to do the job externally from a Windows machine. You need first to enable USB debugging to make that possiblle.
Go into Settings>About Phone then Build Number and tap on that seven times. When you've done that you'll see "you are now a Developer" and in those settings is the option to enable the USB debugging.
That also allow you to transfer files and folders between devices, including Windows systems.