Click the image to see the NASA page about this asteroid.
As the above Aricebo radio telescope image suggests, 2005 YU55 is approximately spherical. It also slowly spins with a rotation period of about 18 hours. The asteroid's surface is darker than charcoal at optical wavelengths, but because it's so big and will be so close, you should be able to see it using a telescope with an aperture of 6" or more. However, at its closest approach it'll be streaking across the sky at about 9° per hour; that's about the size of your fist held at arm's length, so even if you can find it, it'll slide across your eyepiece field of view pretty quickly. Because it's blazing along at 30,000 miles/hour, it'll go from a point source among the outer planets to something you can see in your little 'scope back to a point source among the inner planets in basically two days! Zoom!
If you really want to hunt down this micro-planet with your telescope, this JPL page has lots of useful information.
This is the closest that this Earth-grazer has passed for more than 200 years and the closest approach by such a big asteroid that we've ever known about... in advance, that is (hello, dinosaurs!). In fact, the next time an asteroid of similar size (that we know of) approaches this close to Earth is in 2028. Sadly for doomsday fans, though this asteroid passes within 0.84 of the Earth-Moon distance, it likely won't have any effect on Earthly tides or electronics or anything else, really. Unless, y'know, you happen to be en route to the Moon, in which case you could enjoy a lovely close encounter with this space beast.