Hi Dear, IISER is the topmost government college for education and research and its affiliated with MHRD.there are three ways to get admission on IISER -

1.KVPY 2.JEE 3.SCB(12th)(Aptitude test)

so prepare any exam.

if you want other information then check it out given link:
https://engineering.careers360.com/articles/iiser-admissions

good luck

The same way as you prepare for jee main or other exam you can prepare for iiser.there is test named NEST which is used to get iiser.So prepare for it.

All the best