FreeNAS is an extremely popular NAS operating system. It is designed to make setting up your own NAS easy. It provides a nice web UI with all sorts of bells and whistles. It is also open source. It is based on two main underlying technologies. FreeBSD and OpenZFS.
If you want a NAS with high functionality and you want it to be easy, this probably your best choice.
ZFS is an important technology. It basically a combination of a file system and a volume manager in one. It gives you the ability to configure your disks with different RAID levels. It also provides data integrity which is more important than you might think. It helps protect against the dangers of bit rot. If you care about your data you should probably be using a filesystem like ZFS or BTRFS. Both are great choices but I think BTRFS is supposed to be more mature.
People are sometimes surprised to learn that FreeNAS is based on FreeBSD. People may assume that it is just a NAS oriented Linux distro. It is definitely not Linux based. Why BSD? It is a great system for servers. It also has great support for ZFS. If you want something really stable and rock solid for your storage server, BSD is a good foundation.
You can start out by downloading the ISO HERE
Once you have the ISO, you can boot off of this. You will need to select a disk to install to and a password. You won’t have to do much more. Once it is installed you can reach it on port 80 for HTTP and port 443 for HTTPS. For example, assuming your server’s IP address is 192.168.0.5 you would just enter this URL in your browser to reach the web UI.