At graduation level you can opt out at least one of the following subjects:

Agriculture, Botany, Forestry, Chemistry, Geology, Mathematics, Physics, Statistics, Animal Husbandry or any of the engineering subjects.

You must explore the website of Indian Forestry Service conducted by Union Public Service Commission, Govt of India.

Follow the instruction and see the subject combination, apart from that you need to prepare for the General Studies and General Knowledge.

Please find the important instruction for the preparation :

1. English is the pre-requisite for any forest service, physical fitness is also required and no criminal record according to common service rule.
2. Understand all the subjects 8th to 12th level prefer NCERT for better understand and then draw the conclusion of your subject interest where you are hundred percent comfortable.
3. Accordingly you take up your subject for your graduation level and then get ready for Indian Forest Service which will stamp you for the top level of commissioned officer job which will make you a district forest commissioner or more than that. Even you can serve the nation as the executive secretory for ministry of environment and forest / ministry of renewable energy etc.
4. Above suggested things will lead you to the prime level as a policy maker to do the better job for forest, animals, victim tribes etc.
5. If you fail to attempt the above option -IFS then you can go for the alternative at state level of forest service like district forest ranger where you have to see the state level of policy related to exams and its subject structure.
6. After knowing the pre-requisites at 12th and graduation level start to prepare yourself and do your best.

Based on the above cited structural facts you need to understand the basic and primitive facts while preparing for IFS or PCS -FS which will help you to build up your confidence. Therefore you must explore the forest service life and interact with the forest personal and look at their day to day routine work and then assess yourself whether I am fit for this job or not. In general people make up the mind prematurely and join the irrelevant service in order to satisfy others interest, choice and personal glory. It could be fatal if you are not awared.
Hope you are clear in saying all these things--

Good luck