Dear Aspirant, the eligibility criteria for a biologist is studying biology subjects from 10+2 standard. There are many subjects which comes under biology stream. These are Microbiology, Botany, Zoology, Biotechnology, Biochemistry, Agriculture, Pharmacology, Environmental Sciences, Biomedical Science.
In 10+2 standard you should take biology as one of the subject. In graduation, you should take any one of the biology subject such a botany, zoology, microbiology and other subjects mentioned just above.
In graduation if you study botany and zoology, still you can choose specialized subjects (Microbiology, Biotechnology and others) in post graduation course.
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n post graduation you have to choose a specialized subject for example; botany, zoology, biochemistry, genetics, biotechnology. If you wish to pursue Masters and doctoral program in agricultural subjects (agronomy, plant breeding, horticulture, forestry, Nemtology, Entomology) then you should have studied graduation in Agriculture discipline i.e. BSc Agriculture.

To become a biologist you have to obtain a degree in PhD degree in relevant subject. Normally it happnes that any one obtained masters degree in Botany can pursue doctoral degree in Microbiology, Biotehnology. But it will be better for you to have degrees (masters and doctoral ) in one disciple. This helps in getting job, because the people in selection committiee may raise question of hybrid degree.
After getting the doctoral degree you have look for jobs in university and institutes of private sector. In university or colleges you will be inducted as Assistant Professor or lecturer. In institutes or private sector, you will be recruited as scientist.

You may also opt environmental science or forensic science or biomedical science. All these disciplines are also having great job opportunities in India and abroad.

NOrmally in Indian scenario there is no "Biologist" in a job, you will be known as botanist, zoologist, biochemist, microbiologist, biotechnologist, genetecist, environmental scientist, biomedical engineer.
In agriculture discipline, the biologist or scientist is known by the expertise in his or her field such as Agronomist, Nematologist, Entomologist, Plant Breeder. In western hemisphere, universities award degree in Biology (even in doctoral program).
Therfore, to become a biologist, you have to study biology from 10 standard onwards. You have not written in your question that at present what you are studying.
Hope I have answered your quesry, still you wants to know more, please feel free to ask again.

Good Luck...