Only fragments of Greek philosophy from before Plato survive. Shelves and shelves of books have been written about these fragments, and on interpreting and second-guessing the way that later authors talked about them, but it's ultimately guesswork what Socrates thought, let alone who he learned from - Socrates, after all, didn't write anything down himself. What we do know about Socrates we largely know from three sources, two of which very likely put words into his mouth, and one of which is not particularly in-depth on his philosophy. The first problematic source on Socrates is a play by the comedic playwright Aristophanes, The Clouds, which spends much of the play taking the piss out of Socrates:

STREPSIADES First, what are you doing up there? Tell me, I beseech you.

SOCRATES (POMPOUSLY) I am traversing the air and contemplating the sun.

STREPSIADES Thus it's not on the solid ground, but from the height of this basket, that you slight the gods, if indeed....

SOCRATES I have to suspend my brain and mingle the subtle essence of my mind with this air, which is of the like nature, in order clearly to penetrate the things of heaven. I should have discovered nothing, had I remained on the ground to consider from below the things that are above; for the earth by its force attracts the sap of the mind to itself. It's just the same with the watercress.

Aristophanes is not a careful observer of Socrates' philosophy, it has to be said, and it's probably the case that Aristophanes chose Socrates as a stand-in for philosophers in general. So trying to interpret Socrates through Aristophanes is probably like trying to interpret the 20th century materialist 'Australian school' in the philosophy of mind (e.g., David Armstrong) through Monty Python's Australian philosophers sketch.

Plato, too, is a problematic source for Socrates, most notably because Plato as a philosopher writes in dialogues, rather than straight prose, and the main character in the dialogues is Socrates, even when the ideas being pushed by 'Socrates' in the dialogues are usually considered to be Plato's own, and quite different from anything that the historical Socrates probably said. With that said, there's a traditional divide between what constitutes 'early' Plato, where he's taken to be just recording things Socrates said, and later Plato, where he's putting Platonic words into a Socratic mouth.

Xenophon's Socrates in Memorabilia is again a different Socrates; like Plato's Socrates he engages in dialogues - but where Plato's Socrates pursues arguments to their ultimate end, Xenophon's Socrates is....practical. Xenophon records Socrates in conversation with Nicomachides, who is surprised when Socrates' opinion on the qualities of a good general aren't all about bravery:

NICOMACHIDES: Socrates, I should never have expected to hear you say that a good housekeeper and steward of an estate would make a good general.

Socrates leads Nichomachides up a long series of typically Socratic questioning before concluding:

SOCRATES: Why, it is just then, I presume, it will be of most service, for the good economist knows that nothing is so advantageous or so lucrative as victory in battle, or to put it negatively, nothing so disastrous and expensive as defeat. He will enthusiastically seek out and provide everything conducive to victory, he will painstakingly discover and guard against all that tends to defeat, and when satisfied that all is ready and ripe for victory he will deliver battle energetically, and what is equally important, until the hour of final preparation has arrived, he will be cautious to deliver battle. Do not despise men of economic genius, Nicomachides; the difference between the devotion requisite to private affairs and to affairs of state is merely one of quantity. For the rest the parallel holds strictly, and in this respect pre-eminently, that both are concerned with human instruments: which human beings, moreover, are of one type and temperament, whether we speak of devotion to public affairs or of the administration of private property. To fare well in either case is given to those who know the secret of dealing with humanity, whereas the absence of that knowledge will as certainly imply in either case a fatal note of discord.

So, in summary, there's three different versions of Socrates in our sources, and very little information on who came before Socrates. So who taught Socrates?

One thing that is clear is that the things that philosophers talked about before Socrates were decidedly about things like the nature of reality, what the universe is made of, whether existence is more about change or stasis, things like that - the kind of thing you probably discuss with your friends when you're stoned. They weren't talking about Earthly things like whether the Greek equivalent of Jeeves would make a good general - Socrates was seemingly somewhat original in using abstract philosophical argument to discuss Earthly matters.

I mean, to be honest, because much pre-Socratic philosophy is fragmentary, it's hard to tell sometimes whether the level of discourse is smarter than you and your stoned mates. Here's the oldest existing fragment of philosophy, from Anaximander:

Whence things have their origin, thence also their destruction happens, as is the order of things; for they execute the sentence upon one another - the condemnation for the crime - in conformity with the ordinance of Time.

Perhaps he was listening to Pink Floyd as he said that, and then someone said "whoa, man". I kid, but Socrates is generally considered to find something new in philosophy compared to the 'pre-Socratics'. The other big thing about the pre-Socratics - Thales, Anaximander, Parmenides, Heraclitus, etc - is that very few of them were Athenian; instead, Greek pre-Socratic philosophy seems to have originated in the islands off the coast of modern-day Turkey; other pre-Socratic philosophers were in Greek colonies in Sicily. Anyway, stoned old Anaximander died about 70 years before Socrates was born, so he wasn't Socrates' teacher. The pre-Socratic philosopher who is mentioned in Plato's writing (in the Phaedo in particular) as an influence on Socrates was Anaxagoras, who is generally seen as the person who introduced Athens to philosophy:

Then one day I heard a man reading from a book, as he said, by Anaxagoras, that it is the mind that arranges and causes all things. I was pleased with this theory of cause, and it seemed to me to be somehow right that the mind should be the cause of all things, and I thought, 'If this is so, the mind in arranging things arranges everything and establishes each thing as it is best for it to be. So if anyone wishes to find the cause of the generation or destruction or existence of a particular thing, he must find out what sort of existence, or passive state of any kind, or activity is best for it. And therefore in respect to that particular thing, and other things too, a man need examine nothing but what is best and most excellent; for then he will necessarily know also what is inferior, since the science of both is the same.

As I considered these things I was delighted to think that I had found in Anaxagoras a teacher of the cause of things quite to my mind, and I thought he would tell me whether the earth is flat or round, and when he had told me that, would go on to explain the cause and the necessity of it, and would tell me the nature of the best and why it is best for the earth to be as it is; and if he said the earth was in the center, he would proceed to show that it is best for it to be in the center; and I had made up my mind that if he made those things clear to me, I would no longer yearn for any other kind of cause.

And I had determined that I would find out in the same way about the sun and the moon and the other stars, their relative speed, their revolutions, and their other changes, and why the active or passive condition of each of them is for the best. For I never imagined that, when he said they were ordered by intelligence, he would introduce any other cause for these things than that it it is best for them to be as they are. So I thought when he assigned the cause of each thing and of all things in common he would go on and explain what is best for each and what is good for all in common. I prized my hopes very highly, and I seized the books very eagerly and read them as fast as I could, that I might know as fast as I could about the best and the worst.

My glorious hope, my friend, was quickly snatched away from me. As I went on with my reading I saw that the man made no use of intelligence, and did not assign any real causes for the ordering of things, but mentioned as causes air and ether and water and many other absurdities. And it seemed to me it was very much as if one should say that Socrates does with intelligence whatever he does, and then, in trying to give the causes of the particular thing I do, should say first that I am now sitting here because my body is composed of bones and sinews, and the bones are hard and have joints which divide them and the sinews can be contracted and relaxed and, with the flesh and the skin which contains them all, are laid about the bones; and so, as the bones are hung loose in their ligaments, the sinews, by relaxing and contracting, make me able to bend my limbs now, and that is the cause of my sitting here with my legs bent.