Assalaam alaykum Warahmatullahi wabarkatuhu,
Praise be to Allah SWT and Peace and Blessing be on His messenger pbuh, and i bear witness that there is none worthy of worship except Allah SWT andProphet Muhammad pbuh is his messenger.
Many of our brothers of faith believe in intercession of people who are already dead as a means to approach Allah SWT, but they understand not that Allah SWT does not require any intermissary to listen to our prayers. Allah SWT says in Holy Quran,
And O dear Prophet (Mohammed – peace and blessings be upon him), when My servant question you concerning Me, then surely I am close; I answer the prayer of the supplicant when he calls on Me, so they must obey Me and believe in Me, so that they may attain guidance. 
You alone we worship and from You alone we seek help. 
And indeed We have created man and We know what his soul instils in him; and We are nearer to him than the hearts artery. 
Now, we should know how the Shirk itself started in Human kind after Allah SWT send Adam AS to inhabit the earth.
It is narrated from our beloved Prophet Muhammad pbuh,
“No babe is born but upon Fitra. It is his parents who make him a Jew or a Christian or a Polytheist. …………..” 
“No child is born except on Al−Fitra (Islam) and then his parents make him Jewish, Christian or Magian, as an animal produces a perfect young animal: do you see any part of its body amputated?” Then he rec ‘The religion of pure Islamic Faith (Hanifa),(i.e. to worship none but Allah), The pure Allah’s Islamic nature with which He (Allah) has created mankind. Let There be no change in Allah’s religion (i.e. to join none in Allah’s worship). That is the straight religion; but most of men know not…” 
From the above mentioned hadiths it is clear that humans were born sinless and pure into the Islamic faith, and Prophet Adam pbuh was send to the earth as vicegerent of Allah SWT on earth. When Adam pbuh descended, he taught his off-springs about the acts of worship to Allah SWT and to follow his path. It was in the later generation of Prophet Noah pbuh that the first act of Shirk began. Allah states the following in Quran for the people of the Noah,
“They have devised an outrageous plot, and said to each other: ‘Do not leave your gods; especially Wadd, Suwa, Yaghus, Yauq and Nasr (the names of their idols).’” 
Following is narrated from Ibn Abbas RA refererring to these verse;
“All the idols which were worshipped by the people of Noah were worshipped by the Arabs later on. As for the idol Wadd, it was worshipped by the tribe of Kalb at Daumat− al−Jandal; Suwa’ was the idol of (the tribe of) Murad and then by Ban, Ghutaif at Al−Jurf near Saba; Yauq was the idol of Hamdan, and Nasr was the idol of Himyr, the branch of Dhi−al−Kala.’ The names (of the idols) formerly belonged to some pious men of the people of Noah, and when they died Satan inspired their people to (prepare and place idols at the places where they used to sit, and to call those idols by their names. The people did so, but the idols were not worshipped till those people (who initiated them) had died and the origin of the idols had become obscure, whereupon people began worshipping them.”
In his greatest ‘EPIC’ Tafsir ibn Kathir, Ibn Kathir sheds lights on the above verse,
These are the names of their idols which they used to worship besides Allah. Al-Bukhari recorded from Ibn `Abbas that he said, “The idols that were among the people of Nuh wound up among the Arabs afterwards. In reference to Wadd, it became the idol of the people of Kalb in the area of Dawmat Al-Jandal. Suwa` became the idol of the people of Hudhayl. Yaghuth became the idol of the people of Murad, then the people of Bani Ghutayf at Al-Juruf in the area of Saba’ worshipped it after them. Ya`uq became the idol of the people of Hamdan. Nasr became the idol of the people of Himyar for the family of Dhu Kala`. These idols were all named after righteous men from the people of Nuh. Then when these men died, Shaytan inspired his (Nuh’s) people to erect statues in honor of them at their gathering places where they used to come and sit, and to name these statues after these men (with their names). So they did this (as Shaytan suggested), but these statues were not worshipped until after those people (the ones who built them) had died and the knowledge was lost. Then, those statues were later worshipped.” This has also been similarly reported from `Ikrimah, Ad-Dahhak, Qatadah and Ibn Ishaq. `Ali bin Abi Talhah reported from Ibn `Abbas that he said, “These are statues that were worshipped in the time of Nuh.” Ibn Jarir recorded from Muhammad bin Qays that he said concerning Yaghuth, Ya`uq and Nasr, “They were righteous people between the time of Adam and Nuh, and they had followers who used to adhere to their guidance. Then, when they died, their companions who used to follow them said, `If we make images of them, it will increase our desire to perform worship when we remember them.’ So they made images of them. Then, when those people died and other people came after them, Iblis approached them and said, `They (your predecessors) used to worship these statues and they were granted rain by their worship of them.’ Thus, they (the latter people) worshipped them.
Further the muhaddith of the last century, Shaykh Muhammad Naasir ud-deen Albanee said in one of his writings,
Abu Ja’far al-Baaqir said: “Wadd was a Muslim man who was loved by his people. When he died, the people began to gather around his grave in the land of Baabil ( Babel ), lamenting and mourning. So when Iblees (Satan) saw them mourning and lamenting over him, he took the form of a man and came to them, saying : I see that you are mourning and lamenting over him. So why don’t you make a picture of him (i.e. a statue) and place it in your places of gatherings so that you maybe reminded of him. So they said: Yes, and they made a picture of him and put in their place of gathering; which reminded them of him. When Iblees saw how they were (excessively) remembering him, he said : “Why doesn’t every man amongst you make a similar picture to keep in your own houses, so that you can be (constantly) reminded of him.” So they all said “yes”. So each household made a picture of him, which they adored and venerated and which constantly reminded them of him.
“Those from the later generation saw what the (pevious generation) had done and considered that……..to the extent that they took him as an ilah (diety) to be worshipped besides Allaah. He then said :” This was the first idol worshipped other than Allaah, and they called this idol Wadd” 
Hence, the Shirk began with,
- First, Satan inspired people to make statues of righteous persons from the people of the Nuh after their death to remember them.
- When these people died and their knowledge was lost, Satan approached the new generation and lied unto them that their predecessors used to worship them and that they were granted rain with their worship of them. And thus the worship of these statues started.
In general, the main causes for Shirk to enter the form of worship were, 
- Exaggerating in the remembrance and dignifying of the pious people and extending this remembrance to the extent of worship.
- Distancing from the Book of Allah SWT and the Prophet’s traditions.
- Determination to imitate and follow the ancestors, without verifying their beliefs, despite them straying from the right path of Allah.
- Raising suspicion in answer to the call of the prophets.
- Ignorance among the people that the worship to Allah SWT is complete by merely uttering of the testimony, ‘La-Ilaha-Illallah’.
I believe and hope that this is sufficient to make our brothers and sisters of faith to understand the truth
You alone we worship and from You alone we seek help (and may we always). Guide us on the Straight Path. The path of those whom You have favoured – Not the path of those who earned Your anger – nor of those who are astray. 
1– Surah Baqarah, Verse 186
2– Surah Fatihah, Verse 5
3– Surah Qaf, Verse 16
4– Sahih Muslim, Hadith no. 6426, narrated by Abu Huraira
5 – Sahih Bukhari – Vol 6, no. 298, narrated by Abu Huraira
6 – Surah Nuh, Verse 22 & 23
7 – Sahih Bukhari – Vol 6, no. 442, narrated by Ibn Abbas
8– TAFSIR IBN KATHIR – http://www.tafsir.com/default.asp?sid=71&tid=55472
9 – ORIGIN OF SHIRK – http://www.alalbany.net/albany_eng_030.php
10 – AN EXPLANATION TO THE TRUTH OF MONOTHEISM by Sheikh Salih al-Fawzaan
11 – Surah Fatihah,
- Protecting your Emaan by Nisaar Y. Nadiadwala (xeniagreekmuslimah.wordpress.com)