When Buddhism started? After Gautama Buddha's born or Buddhism were already there in the world when Gautama Buddha was born? who was the first Buddhism follower?
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 &amp; 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).