The comet is already too low for reasonable viewing at the end of civil twilight, so your best bet is to start looking immediately after sunset. Start scanning with binoculars immediately above the spot where the Sun disappeared, and then move slightly to the right as time passes. While you're at it, see how soon you can pick up Venus. If you can find the exact right spot, it should be visible to the unaided eye as soon as the Sun is gone.
If you own a Go To telescope, you can obtain R.A. and Dec. from the Minor Planet Center. Note that the data is given for 0h Universal Time, which corresponds to evening on the previous day in North America. So use the ephemeris for January 13th to see the comet on the evening of the 12th.
Up to now, people at far northerly latitudes have had the best views of Comet McNaught, but on January 12th the observing geometry is pretty much the same throughout the North Temperate Zone. On the 13th people at low latitudes in both hemispheres will be best placed to see the comet, and after that, people in the Southern Hemisphere will be favored.
On the evening of January 14th, observers at mid-southern latitudes may see the comet emerge very low in the sky immediately after sunset right next to the planet Mercury. This conjunction should be impressive but tricky to observe. Every evening after that the comet will climb rapidly higher, as shown at right, and soon it will be visible in a fully dark sky. But the comet is likely to be fading rapidly as well. Stay tuned for ongoing developments!
Check out our photo gallery for some lovely pictures of the comet, and feel free to submit your own. You can also e-mail observing reports to observers@SkyandTelescope.com. And check out our own observations, including a remarkable image of the comet's head and tail taken in broad daylight.