I need to change my filesystem to the NTFS filesystem from ext4, but the disks tool reports that the current disk is in use, which is obvious. How can I format my disk using terminal before booting? Or how do I go around this error?
Boot into a live environment from a CD / USB drive and format your disk from there.
Also obviously make sure you copy all of your data off first and have backups, you can't "change" a filesystem, only completely remove one and replace it with another empty one.