Hello,

First of all the eligibility criteria for appearing in Indian Forest Service is passed Bachelor's degree having studied any one of the subjects amidst Animal Husbandry & Veterinary Science, Botany, Chemistry, Geology, Mathematics, Physics, Statistics and Zoology or a Bachelor's degree in Agriculture/Forestry/ Engineering , if you wish to appear for IFS then you should have graduation in any of the aforementioned discipline as per your preference, next you're still in 11th standard, so there are still 4-5 years to go depending upon the duration of your graduation, see this exam is highly competitive, and you're still young, you may want to appear for this exam that's a good thing, but it's important to sustain the motivation, and for that you need to be extremely clear about your career goals, since this is a long journey it takes a minimum of two years in the sense you'll be preparing for one whole year, and the entire examination of prelims, mains and interview happen over a period of one year, so you need to have single minded focus, dedication and perseverance to qualify this exam, at this point I will suggest focus only on your studies, and once you enter college try to dedicate yourself in knowing this exam, such as go through syllabus to learn it by heart, this will enable you to understand what to read, what to skip, develop the habit of newspaper reading this is crucial from the point of view of prelims/mains/interview, and you shouldn't stop this habit ever, next start off with NCERT and then move towards other standard books, further go through previous year papers, apply mock tests on regular basis to analyze yourself in terms of preparation level, and practice answer writing in advance, these are some of the general overview I've mentioned in regards to preparation of the exam, but as I said your priority at this time should be your studies, focus on it whole-heartedly, and you can appear for IFS even after  completion of graduation, see there are many aspirant coming from diverse fields in some cases they also have relevant years  of experiences in their domain, in case your goals regarding UPSC isn't clear you can explore that sphere of work as well.