Science

No, you can go for IISER after 12th. First, be sure your dead set on IISER. Research is not what is taught in school. Its hard, painstaking and snail speed slow. There are no gigantic leaps, rather a collection of tiny steps. If your dead set, then I recommend preparing for the KVPY examination -. http://kvpy.iisc.ernet.in/main/eligibility.htm
- Getting a good rank in SA or SX stream almost guarantees you a seat at IISER. If you don't qualify, don't be disheartened there exist other avenues. Take the IIT-JEE Advanced and get a rank below 5000 - that alsoalmostguarantees you a seat at IISER. If you still don't qualify, don't be disheartened. Do well in your board examinations (12th). Earlier it used to be top 1 percentile, but now they have relaxed the criteria. The minimum percentage for this now is listed here -https://www.iiseradmission.in/?page_id=8. But these are bound to change (guessing, increase? ). In the next 2 years. So, do well there. But this is just the eligibility criteria. To gain admission, you will have to sit for the IISER Aptitude Test - similar to KVPY tests. Your rank in that test will determine your chances for admission. But that's it. There are only three avenues to get into IISERs. As a side note, Id ask you to apply to IISc as well.