Why does Hamlet tell Ophelia to go "to a nunnery" and what does he give as his reason?

When he states "get thee to a nunnery" to Ophelia, he is expressing pent-up anger towards his mother, who he feels has been unfaithful and incestuous when she married his uncle.  At the beginning of the play itself, we see a brooding Hamlet who seems almost more upset by his mother's marriage than by his father's death.  He speaks of it with such bitter disgust:  "She married, O, most wicked speed, to post with such dexterity to incestuous sheets!" (I.ii.156-7) and is so upset with his mother that he pronounces a curse on ALL women, not just her:  "Frailty, thy name is woman!" (I.ii.147).  So, that curse includes Ophelia, and from that point on, he avoids her.  Then, when she confronts him, he lets out a huge rant on all women in general-it is a way to indirectly vent his rage at his mother, since he suspects she is listening.

So, think of a nunnery.  There, women cannot marry at all; they cannot be under the influence of any men, or influence men in any way.  Hamlet feels that is where a woman has a best chance at being faithful, and where she will cause the least amount of damage.  After all, as he tells Ophelia also, "why wouldst thou be a breeder of sinners?"; in a nunnery, she won't have children and bear wicked men-like his uncle-that do awful things.  A nunnery will keep her from marrying, but if she were to marry, Hamlet says, "be thou as chaste as ice, as pure as snow...[or] marry a fool" who doesn't know "what monsters you make of them" (III.i.122-146).

I hope that explanation helps a bit!  Good luck!