Hi,

IES is conducted the following - 

Category I Civil Engineering.  

Category II ‐Mechanical Engineering.  

Category III ‐Electrical Engineering.

Category IV ‐Electronics & Telecommunication Engineering.

The detailed syllabus and other information can be checked here http://www.upsc.gov.in/sites/default/files/Notification_ESE_2018_Engl_Revised.pdf

Match the syllabus with your engineering one and study accordingly. Otherwise it will be tough to study for the specialized subjects. My advice would be to start from 2nd year as the your core engineering subjects start then. 

Cutoffs which are the minimum marks you require for the shortlisting. so check that out here - http://www.upsc.gov.in/sites/default/files/ENGG_2017_CutOffs_English.pdf

Past year papers - http://www.upsc.gov.in/examinations/previous-question-papers. This will help you know the type of questions you will be asked.

Follow the exam details every year since you have 4 years before you will attempt this. Practise from sample papers. there are plenty available in the market.