Forestry

Both are good career option and have it's own importance in the society. You have to choose in which you have more interest or suits you the most. Horticulture is about practice of growing flowers, fruits, etc. While forestry is the study of conserving & managing forests. In both you are going to take care of nature, but which is better for you that you have to decide. By the way forestry has more scope.

Just open site of any forestry college and fill all your information like name, class 12th marks, address, etc.

After graduation, you can give different government exams like forest range officer and other jobs in forest department and you can also get job in private companies with a good package up to INR. 70,000 per month.

Yes, NLSIU PG course is only available through CLAT PG exam. Otherwise, you can apply to different universities. Also, I doubt that NLSIU offers PG in international environmental law.