(Serious answer)

The first human to be considered a deity, both in nature and by contemporaries, would be an Egyptian pharaoh. Which one is however debatable. Presumably, the first pharaoh of Dynasty 0 should be the first human to be considered a deity (that we know of); but whether that is King Scorpion, Narmer or another is still debated to my knowledge.

The first pharaoh that we can reliably tell was de facto worshipped as a god while living is Djoser of the 3rd Dynasty. Because of the way ancient Egyptian religion works, I would claim it reasonable for this practise to have appeared long before Djoser started his pyramid build, but alas, his construction work is the first that more definitely points us in the direction of a building being built to serve as a tomb for the body of a god. Arguably though, you could claim that the existance of Horus names for earlier pharaohs would be enough evidence for their in-life-divinity.

However, if you're looking for someone who's been deified during their lifetime, rather than being born a god, you have to look at the extended time period after the Trojan War. The Trojan War itself is an important breakpoint in this part of history, because so many difference people claim descendance from heroes who supposedly (read: in The Iliad) took part in the war.

Two possible candidates would thus be Alexander the Great, who was said to be descended from Zeus, or Scipio Africanus. The issue with both of these, is that they weren't considered deities by their own people. Alexander the Great adopted a style that the great Persian kings had come to use, which seemed to the Greeks as if he tried to promote himself as a god. And in that sense, you do get an answer to your question in that it was not well taken by his contemporaries. Scipio, likewise, wasn't seen as any sort of god by his contemporary Romans. It is actually Polybios who tells us that:

But this same loftiness of character was still more admirable in the closing scenes of his life, when, in addition to his achievements in Iberia, he crushed the Carthaginians; reduced the largest and fairest districts of Libya, from the Altars of Philaenus to the Pillars of Hercules, under the power of his country; conquered Asia and the kings of Syria; made the best and largest part of the world subject to Rome; and in doing so had numerous opportunities of acquiring regal sway, in whatever parts of the world suited his purpose or wish. For such achievements were enough to have kindled pride, not merely in any human breast, but even, if I may say so without irreverence, in that of a god.

Polybios is telling us about how the Iberians, after Scipio has freed them from the Carthagians, starts revering him as a king and it's actually he (Polybios) who subsequently claims that, because of the Iberians' familiarity with Eastern king-cults, they should most probably rever Scipio as a god. It is actually unclear whether they really did so or not, but more importantly and relevant to your question, the contemporary Romans did not view him as such.

To get a clear example of a person who was born a mortal human being, but was deified when he died because of his godly status, you have to look as late as Julius Caesar. And even he wasn't considered a god while he was still alive; though he wasn't contested as a god by his contemporaries after he died.

There is still a bit of a debate amongst Galactic scholars on that topic. Certainly, as the Kwisatz Haderach, the first male to survive the spice agony, it can be said that Paul Atreides was "above humans" but then again the most convincing argument against that is that it basically only allowed him access to the Other Memories, something the Bene Gesserit had been able to do for ages.

If you want to go with a pretty certain pick it is of course Paul's son and successor (bar the regent-turned-pretender episode of his sister Alia), the Emperor Leto II Atreides, mostly known as "God-Emperor" or "Tyrant". Not only was he pre-born and thus able to access the Other Memories (more successfully than his Aunt), ever since he entered a symbiotic relation with the sandtrouts and effectively became a sentient sandworm himself he was for all intents and purposes immortal and all-knowing, constantly striving to mold the Empire according to his Golden Path.

Did he go unchallenged? Of course not, but no assassin ever managed to harm him and not even the great Duncan Idaho, many times reborn as a ghoula, could escape the God-Emperor's prescience until the very end when the Atreides breeding line managed to become invisible to prescience.
Large-scale revolts were also quenched easily due to the Tyrant's total control over the vital Spice market (coming with huge political leverage) and his devout army of Fish Speakers.

Sources:

• Princess Irulan's Private Reflections on Muad'Dib and Arrakis Awakens

• Various texts from the Bene Gesserit Archives (unfortunately behind a pay-wall)

• Leto II.'s private chronicles commonly known as The Stolen Journals, and lastly the Oral History (although one needs to be careful with those last two texts as their contents were clearly influenced by the Tyrant and his Fish Speakers for propaganda reasons)

Eärendil the Mariner. While he is often only called half-human, being the son of Tuor and Idril Celebrindal, such racist conceptions of a strictly racial division between the Firstborn and Second children are no longer modern. Advances in biological theory have totally overturned former racist concepts of mediterranean history, treating both groups (including Hobbits) as one species. Phenotypical differences are much more a result of different environmental factors, and in any case the difference between Avari and Vanyar is larger than between Noldo and Edain, for example.

The 'Song of Eärendil' has an important passage here:

the Silmaril as lantern light and banner bright with living flame to gleam thereon by Elbereth herself was set, who thither came and wings immortal made for him, and laid on him undying doom, to sail the shoreless skies and come behind the Sun and light of Moon.

Eärendil is here directly bestowed divine attributes, by none other than the Valië Elbereth. He is essentially become a divine being himself.

I realize that my view might be unpopular, especially with more old-fashioned scholars, and it certainly is possible to arrive at other interpretations. Some theologians of the old school might not agree with this, but it would be fair to call Eärendil at least divine. He is a divine messenger, a herald star, comparable to Hermes or Mercur, and at the same time, he is also being called upon as a benign deity, for example by Frodo. He is a divine symbol of light and hope, especially for people in later times of the Third Age. At the very least, he is divine, as angels/elohim are in Christian mythology of later times.

Some others have argued that the first would be Beren Erchamion, who was much revered in Ossiriand at the time, but his cult wasn't very long-lived and we do not know much about it.

While the apotheosis of Talos is perhaps more famous, it was preceded long before by the apotheosis of Arkay. Maybe.

The origins of Arkay is a hotly debated topic among theologians. Both Mymophonus and Mannimarco (in Arkay the God and Arkay the Enemy, respectively) state that Arkay was once a mortal man, despite having otherwise theologically opposing positions. However, The Monomyth, the author of which is unknown but believed to have been a member of the Temple Zero Society, claims that Arkay was among the first spirits to crystallize into a recognizable form following the formation of Akatosh and the beginning of time itself. This parallels other traditions that states that Arkay is the child of Akatosh, though these paradoxically state that Arkay's mother was a moral woman - perhaps in an attempt to reconcile with the tradition of the mortal Arkay.

We might attempt to further reconcile these discrepancies by considering the possibility that Arkay was one of the Ehlnofey, the semi-divine common ancestors of Men and Mer. Alternatively, the current Arkay might be a mortal man who Mantled the previous Arkay and assumed the role of God of Birth and Death.

Of course, if recently developments in the field of c0da studies are accurate, there may be no need to reconcile these disparate versions of Arkay at all.

Back in the early years, long before the Great War, the Antagonists worshipped their false god through disgusting practises and unholy appearances by their so-called priests. But the just and unquestionably real Lord God sent along a man known to us only as the Hanged Redeemer, to save mankind from the sins comitted by the Antagonists.

The Hanged Redeemer, not yet hanged, was tempted by Satan with power over all the world (p.14 in the Holy Book, v.2). The Holy Redeemer, blessed be his name, declined the offer and stayed a mortal soul in the lands of the living. The lands of the living were inhabited by men, which Lord God had created from the rib of Satan, and for that reason the living were unruly to no fault of their own. Satan, who disapproved of God taking the rib from him, made them so.

In order to try to fix his mistake, God sent several prophets to the lands of the living, to tell mankind of his mistake and to inform them of Him and his making and to attempt to bring out all the good in the living. However, these prophets all failed their task and so the Lord God felt compelled to sent his own son to the realm of the living, to fulfill the task that prophets had failed - but they hanged him! And so the son of God became the Hanged Redeember, the first and only divinity to walk in the realm of the living.