As I'm the only person who uses mine I just have an admin account. But for every day use it may be best if you set up two non-admin accounts for yourself and your wife, and only use the admin account for once-in-a-while settings changes, firmware, and other stuff.
For the initial set up, go to
http://start.qnap.com and take it from there. I did a "Local Installation" on that page. I greatly recommend downloading the Qfinder program from the QNAP website. It's a utility that makes it a lot easier to access the NAS and to set things up (network drives, etc).
I believe that newer QNAP NAS can be installed by connecting a mouse/keyboard/monitor to it, but the TS-251 and TS-451 do not have this firmware. When I tried to set my TS-251 this way I just got a "Please set up your NAS" message.