Using an external USB Stick

I am brand new to using a Pi so I need detailed instructions.

What I want to be able to do is take my USB stick out of my pi, plug it into my mac, transfer files to the usb stick, then plug it back into the pi and have it show up in Next Cloud.

Currently my Mac cannot read the usb stick I have because of the format.
I tried formating a separate USB stick as fat32 but when I plug that into the pi the nextcloud server crashes.

So yeah what do I do? I also need step by step instructions since I have no idea what the hell I am doing.