Hii!
To become an Indian Forest Service officer, one must have completed graduation degree in science stream from a recognised university having one of the subjects such as botany, zoology, geology, chemistry, mathematics, physics, veterinary sciences, statistics, etc., You need to clear IFS exam too. So you can decide from the above about your subject choices in order to become one.
You can also study, agriculture, horticulture, chemical engineering, forestry, civil engineering, etc.,
Hope this helps.
Good luck!