How does evolution know what it needs to change?

It doesn't "know" anything... mutations just happen, and then there's natural selection and drift.

Let's imagine that the first giraffes happened to live in a land where fresh leaves lied high on the trees and were difficult to reach. The conclusion is that only the tallest ones would have been able to feed themselves and survive to pass on their genes.

It could also have happened that among the first group of giraffes there were tall and short ones, but after a catastrophe only a few of them survived, and by chance they happened to be (on average) taller than the ones that died. After a few generations, the taller ones might have specialized in feeding on leaves that grew mostly in tall trees.

Of course things are more complicated. Why do most animals (except for sponges and jellyfish) have a very similar body plan? Flies, much like us, also have limbs, eyes, a head, organs that resemble ours... perhaps the genes that determine our body plans are highly conserved? Scientists are still trying to answer these questions.

Evolution doesn't "know" anything.  Most species evolve through natural selection, which is a subtractive process in which less-fit members of a population leave behind fewer offspring than more-fit members. 

It's hit or miss with evolution.  Basically if what evolves has no value, that line won't go any further.  But if it DOES have a valuable function, it will remain.

Evolution is a very slow process, where the "most fit" of any given species survive, and the rest of their kind perish before producing offspring.  For example, the saber-tooth cat, has no living descendants, but several of its "feline cousins" evolved over millions of years to produce the various cats that are alive today.  In many cases, humans kill off predatory species that are a threat to them. Take for example the short-faced bear, that lived in North America at the same time as the early Native Americans.  The short-faced bear was as large as a Kodiak, and it could run close to 40 mph.  There's no way in hell, that "people" (with sticks, rocks, and bow & arrows) could co-exist with such a formidable predator. So, they killed it off. 

We were NEVER worms - we have only ever been humans. Evolution cannot look forward and choose particular traits. It's simply that the best adapted individuals to a specific environment will reproduce more than those individuals who aren't as well genetically adapted. Evolution doesn't KNOW anything, it's a shotgun approach involving random gene mutations followed by non-random selection of certain indiviuals in a given population to reproduce more than others. Given this, add a few million years, and here we are now!

Evolution is a choice by nature.Adapted creatures can survive.Only adapted ones can better survive and reproduce.HOW DOES IT KNOW? Nature knows.

will answer it , what needs changing , give us an idea , even a hint .

It happens the other way round.  Giraffes with shorter necks didn't get as much food and either died or didn't have as many offspring, and ones with longer necks passed on their genes more successfully.  There are many advantageous traits which never evolve simply because they don't exist in the right populations.

"...how does evolution KNOW to grow the neck? I asked him and he answered “If short necked giraffes and long necked giraffes lived in the same environment, which one would survive  if there were tall trees?” which didn’t answer my question..."

Yes, it DID answer your question.  You simply didn't understand the answer.  Others below have tried to explain it to you.  All I'm gonna add is evolution doesn't "know" anything.  Evolution is the RESULT of interaction between a population's inherent, random genetic variation and the prevailing environment, with the environmental conditions acting to select which genetic variants go on to reproduce and which ones don't. That RESULT ends up being non-random, but NOT by any kind of design or deliberation.

Your biology teacher should rethink his answer.  Have a look at all the short-necked antelopes and okapi and such.  They survived great.  So did elephants. ... and so did the ancestors of humans.