Hi Harshit, 

Yes you can start preapring for CAT exam but you will be eligible to give CAT in final year i.e around Nov 2019 in 5th term. Since you have lot of time you can start be joining any good coaching institute that would last for around 10 months. Still you will be having around 6 months to prepare. In thath time you can join any test series. This would help you to increase your speed as speed is equally important as knowledge. 

I would suggest you to start building habit of reading newspaer like The Hindu or Economic Times as it would help you in some entrance exams like IIFT and for interviews also.

All the Best