B.Sc Botany

Please refer to the link below for eligibility criteria:
https://www.shiksha.com/teaching-education/exams/Kerala-set.

Yes, you can go for M.Sc in forensic Science and criminology, but only after after clearing it's entrance exam.

You have a wide variety of choices, you can go for specialisation in botany, agricultural science, forestry, biotechnology, environmental science, nutrition, chemistry , biochemistry. There are few other options as well which include the course related to the given subjects.

Take a look for Professional courses after B.Sc. -
M.Sc (physics, chemistry, maths, biology, botany, geology and zoology, Applied Biology, Bio-chemistry, Bio-technology, Molecular Biology, Genetics, Environmental Biology)
Post Graduate Courses - You can apply for Research based institutes for PG courses. Ph.D. go for integrated Ph.D. programme or prepare for csir NET. IISER - The value of IISER is really high in Science field. Regards.

Please refer to the link provided below for a detailed list of colleges which offer courses of higher studies in B.Sc agriculture in India. You should be able to download their brochures which contains details regarding entrance exams conducted by the authorities in a particular institution.
https://www.shiksha.com/science/colleges/m-sc-agriculture-colleges-india

The OUAT consists of questions from Maths, Physics, chemistry and bIology. Please refer to the link provided below for details regarding exam pattern and marks distribution for different subjects. You should plan to prepare for your exams accordingly, and study subjects which you are strong in, and also improve on subjects in which you think your concepts are not that clear. Make a proper time table and eat, sleep and follow healthy habits during your preparation.
https://www.shiksha.com/science/exams/ouat-entrance-exam/pattern