What are the chances of an unknown asteroid hitting the earth

i have read about ones that have nealy hit the earth before and nobody knew they were coming
it really scares me knowing that one could hit at any time and we would have no warning

By an implicit definition, an asteroid is anything more than 5 m. Earth is often "hit" by these things and, very often, we "discover" them as they hit.
(hitting our atmosphere is considered a hit).

The bigger they are, the easier they are to detect. Anything more than 200 m across (700 feet) would be detected years before hitting.

Here is the list of the bigger ones.
http://neo.jpl.nasa.gov/risk/

Check out the last column in both tables. The Torino scale (name of the city where the scale was established) is a 0 to 10 scale, that combines the probability of a hit, with the consequences of that hit if it were to happen.

The score is 0 if the probability is very close to zero (in statistics, absolute zero is just about impossible). The score will remain zero even if the asteroid is certain to hit, but if the asteroid is too small to cause any serious damage (meaning = more than one house).

We get nervous when the score reaches 4.

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Here is the list of those who pass very close:
http://neo.jpl.nasa.gov/ca/

The first list are the ones that have already passed; the ones with 2011 as the beginning of the "name" were discovered just as they passed (or very shortly before). Notice that they are small (the big one was discovered years ago)

The second list are the ones we know will pass close in the next few weeks. Look at the distance in "LD" (lunar distance). If that number is bigger than 1, then the asteroid will pass further away than the distance to the Moon.

Pretty much zero. The ones that were discovered "after the fact" are quite small - meters across. Just about anything over 15 meters in size is quite easily found by routine searches.