Cindi, the Vivi Nova is a tank that comes with 3 "atomizer" heads. The 3 heads are in 3 different resistance levels - 1.8ohm, 2.4ohm and 2.8ohm. (Those numbers are etched on the heads so you'll know which is which.) The 2.4ohm and 2.8ohm work best on the Vivi Nova, although the 1.8 will work too (it will just drain the battery a little faster).
The atomizer heads just screw on and off the center "stem" of the base of the Vivi Nova. See this pic.
GotVapes VISION ViVi NOVA 3.5ml V2.5 REBUILDABLE TANK [gv-VIVINOVA-TANKV2.5] - $13.95 : GotVapes.com, E-cigarette Supplies - Atomizers Cartomizers Mods Juice and more
It may seem complicated, but it's VERY simple. You just screw the base into the bottom of the tank, fill it with juice (about 2/3 or 3/4 full), screw the top on, let the juice soak into the wicks for a few minutes, and vape your face off. No leaks, no fiddling, no fuss.
When the juice gets low, just unscrew the top and re-fill it. If you want to change juices (or if your juice gets gunky and you just want to clean the Vivi Nova), just unscrew the top and the base from the tank, rinse it all out, dry it off with a paper towell, blow the water out of the center stem, screw it all back together, re-fill it, and vape it.