Hello Biswajit,

You will have to clear the Forest Services Exam of the respective state. It is conducted by the state public service commission. As the vacancies are not as much as other administrative services, the notification for exam comes only after 2-3 years. The exam is different from civil services in many ways. Only science stream graduates can apply for the exam. 

To get a rough idea about the exam pattern you can check the pattern of Indian Forest services exam. Although unlike IFoS, state services exams(which are two-phased) are only one phased. They are conducted in accordance with Entrance and Training Rules of 2004 for SFS formulated by Central Government. Also in some States, it is MCQ based.

Hope this helps.