Q: I wanto to move my itunes library to external drive

You can follow these steps:

Manage and back up your iTunes media library - Apple Support

How to move your iTunes library to a new computer - Apple Support

Moving a library: Quick answer if you use iTunes' default preferences settings:  Copy the entire iTunes folder (and in doing so all its sub-folders and files) intact to the other drive, hold down the option/alt key (shift on Windows) while starting iTunes, and when requested to select a library select the iTunes Library.itl file in the copied folder.  https://support.apple.com/HT201596  The drive will always have to be on and mounted before starting iTunes or you will have to repeat the option-start method.

Follow Limnos instructions.

I highly recommend you do NOT simply move the iTunes media folder.