Resize disk

I have three partitions in my entire HDD, all of which are automatically created by Ubuntu - /dev/sda1 file system ext4 - Mount point / Size - 148 GB Flags - Boot. The other two partions are /dev/sda2 and /dev/sda5 and are of size 1013 MB each and there are no mount points shown for either of them and also no flags.

Now, when I tried to reduce the size of sda1 using GParted, I am getting the following message -

"Could not unmount /dev/sda1. The partition could not be unmounted from the following mount points : /
Most likely other partitions are also mounted on these mount points. You are advised to unmount them manually".

My aim is to get some free disk space, which can be converted to NTFS for installation of Windows. I tried to install windows with the present condition but it is not recognising the hard disk at all.

Can anyone help, please?