After Gautama Buddha's born or Buddhism were already there in the world when Gautama Buddha was born? who was the first Buddhism follower?
Exact Origin year of Buddhism is debated. Some says it began after the death of the buddha while some others believe it started from the BODHI DAY. There are also views stating that it started with the his first teaching to his 5 followers.
When king Suddhodhana came to know about his son`s mortification desires, he sent around 300 men to go with Gautama(Later Buddha) and attend to his needs. But Gautama declined and took only five of them. https://archive.org/details/lifeofbuddhaearl00rock
Also, the argument that his Father is sent men to accompany Gautama is debated but the claim of 5 men accompanying Gautama is verified from almost every source. These 5 men were the first men to which Buddha gave his teachings after the Enlightenment. The names of these are claimed to be as follows:-
1) Kaundinya 2) Assaji 3) Bhaddiya 4) Vappa 5) Mahanama
Together they were known as Pancavaggiya ( The Group of Five ) or Pancaka Bhadravargiya ( The Group of Five Fortunate Ones ).
Kaundinya is said to be the first disciple of Lord Buddha. Kaundinya is the one who predicted that Siddhartha would become Lord Buddha and Kaundinya was the first disciple of Buddha to become arahant.
According to tradition, Gautama Buddha was not the first Buddha. There were previous Samma Sambuddhas before him who taught the same teachings (called Dhamma). Also according to tradition, a new Samma Sambuddha does not appear until the teachings of the previous one has been completely lost and forgotten. According to The Buddhist Society page here:
Once upon a time, many thousands of years ago, there was a Brahmin ascetic named Sumedha living in the Himalayan foothills. He was well advanced in his ascetic and meditation practices. It was the time of the previous Buddha Dipankara. When he heard that the Buddha was to visit the lowlands, he made his way down from his hermitage in order to meet the Buddha.
He helped the people in making preparations for welcoming Buddha Dipankara. When he met the Buddha, the ascetic Sumedha was very impressed and thought, ’May I be able one day to attain Enlightenment like this Buddha, and help the people by teaching the Dhamma’. Buddha Dipankara read the mind of Ascetic Sumedha, and announced to those gathered there, ‘ This ascetic will one day become Enlightened as a Samma Sambuddha named Gotama Buddha’.
A brief history of Gautama Buddha's life can be found here.
The Buddha also described his noble search for enlightenment in the Ariyapariyesana Sutta, including the shramanic (ascetic) teachers he studied under, before his enlightenment, but wasn't satisfied by their teachings. The first disciples were a group of five monks (according to this sutta):
"Then the thought occurred to me, 'To whom should I teach the Dhamma first? Who will quickly understand this Dhamma?' Then the thought occurred to me, 'They were very helpful to me, the group of five monks who attended to me when I was resolute in exertion. What if I were to teach them the Dhamma first?' Then the thought occurred to me, 'Where are the group of five monks staying now?' And with the divine eye, purified & surpassing the human, I saw that they were staying near Varanasi in the Deer Park at Isipatana.
According to accesstoinsight.org's page on the Chronology of Theravada Buddhism:
Year 1 of the Buddhist Era calendar is the year of the Buddha's Parinibbana (death and final release), which occurred in the Buddha's eightieth year (480 BCE according to the "historical" timeline; 544 BCE by tradition).
On this year, the following happened:
Parinibbana (Skt: Parinirvana; death and final release) of the Buddha, at Kusinara (now Kusinagar, India) (age 80).
During the rains retreat following the Buddha's Parinibbana, the First Council (sangayana) convenes at Rajagaha, India, during which 500 arahant bhikkhus, led by Ven. Mahakassapa, gather to recite the entire body of the Buddha's teachings. The recitation of the Vinaya by Ven. Upali becomes accepted as the Vinaya Pitaka; the recitation of the Dhamma by Ven. Ananda becomes established as the Sutta Pitaka.
We can consider this the beginning of modern Buddhism as we know it. Of course, years later, it would split into various schools and traditions.
My personal opinion is that Buddhism started the day the Buddha accepted the invitation by Brahma Sahampati to teach the world (see Ariyapariyesana Sutta).
It's about 150-200 years, that Wester lay people and scholars invented and defined "Buddhism" and the word appears, if remembering right, first in the Encyclopedia Britannica.
If asking about Buddhas teaching, called Buddha- Sāsana, in the origin/traditional countries, it started with the setting in motion the Wheel of Dhamma, short after Buddhas awakining, at least on request of the King od Devas.
The first follower have been some lay people on his way to search for some who are capable to understand the Dhamma, it was before the setting in motion.
On the occation of setting in motion the Dhamma, he accepted (ordained) his first disciples and this was the beginning of his religion he called him self "This Dhamma, this Vinaya", usually called Dhamma-Vinaya (Teaching/Doctrine-Conduct/Discipline).
References to Suttas Names detail: A Sketch of the Buddha's Life gives a good and most relayable short overview of what is general known and approved since this time.
(Note: This is a gift of Dhamma, not meant for commercial use or other wordily gains.)
A good book to read. I highly recommend it for you. This is the book help me to get into Buddhism.
He realized that his spiritual aspirations were far higher than those under whom he chose to learn. He realized that there was none capable enough to teach him what he yearned for – the highest Truth. He also realized that the highest Truth is to be found within oneself and ceased to seek external aid.