how to create my own cloud server on openwrt.
Kinda vague, but the short answer is you don't.
Openwrt is not really suited/tested as a serious cloud platform host, for this you want a x86_64 host with a specialized and tested environment for stuff like Owncloud/Minio or other cloud services.
PS: Otherwise the easiest way to get a simple fileshare, would be to setup a nfs/smb share and use VPN to access the shares via WAN.
Use a LXC container or a chroot with debian and install nextcloud inside it ...