NAS what's the easiest way to do it

Certainly not cheapest, but definately really easy - you can look at Synology or Drobo (and other NAS vendors) for out of the box experience. I'm very happy with my Synology and definately recommend it.

Not sure if this is easiest of cheapest but it's a nice setup:

Lian-Li PC-Q25
300W SFX PSU (FSP GHS 300)
ASUS C60M1-I motherboard (6 x SATA with built-in CPU/GPU)
4GB memory
Up to 5 x 3.5" HDD + 1 x 2.5" HDD

Buy the biggest case you can afford with as many drive bays as possible, fill it up with 3 or 4TB drives, slap in a $30 celeron and $70 B75 board and install a decent server OS (I'd recommend Debian). Set it up (Gigabit preferred) and you are good to go. I'd avoid a NAS, unless you buy one that has everything you could possible need, they are not expandable at all.

Started a 2nd server from purchases on Black Friday:

Case: Fractal Design Arc Midi, 8 HDD bays, 2x 5.25in external
MB: intel DZ68BC 4x SATA3 4x SATA2
CPU: i3-2100T
8GB Crucial DDR3
Highpoint RR2320 8x SATA2
PSU: Seasonic SS-350ET
2x Rosewill 140mm PWM fans

That Extreme Series intel MB was an opportunistic purchase and has 4x SATA3+4x SATA2 ports. Adding a Highpoint RR2320 card gives this server 16 ports capability. I will add a 6x 2.5in in a single 5.25in bay housing giving this box 14 HDD capability, 8x 3.5in+6x 2.5in. The OS SSD will just be wedged somewhere in the case. A multi-card reader occupies the 2nd 5.25in slot.

The Seasonic PSU cables are too short so 24pin-24pin, 4pin 12V cable extensions were added. Fractal Design fan control was too primitive. Rear and top fans were replaced with Rosewill 140mm PWM fans. Original fans were moved to the front and connected to a thermistor equipped fan controller.

Throw a couple/few drives in whatever computer/htpc you already have and share whatever folders you want on them. Keep it simple.

This is best place to start and learn