The most common words taught in first grade are usually pronouns, simply relationship words and two-letter words. The NSES Reading Blog has a list of 100 high frequency sight words for first graders that meets Common Core standards. These words very often appear in first grade level books.
The first 50 words on the most frequently taught sight words list according to NSES Reading Blog are: I, look, you, a, to, have, this, mom, mop, dog, said, what, she, with, in, why, can, am, the, see, seed, feed, we, are, do, dad, come, my, little, he, me, stop, on, and, play, like, go, is, for, forget, it, cat, cut, here, at, was, has, where, went and her. The second half of the first grade sight words are: yes, all, call, calling, down, run, his, came, there, no, out, big, bag, ran, saw, get, got, not, up, make, makes, your, him, old, will, day, from, us, after, who, eat, now, were, house, then, want, use, of, jump, jumped, they, one, or, could, should, good, them, but, over, too, some, something, put, girl, girls, boy, because, car, star, start, when, first, quick, quickly, school, very, new, flew, friend, know, around, most, under, before, off, walk, about, these, other, inside, every and our.
These words have no order of importance and should be automatically recognized by children exiting first grade. The NSES Reading Blog offers free, printable lists of 100 sight words for grades up to fourth grade, as well as printable sight word flash cards.