Hi Omprakash, 

Since you are currently in your 2nd year of graduation, you will be eligible to write CAT from next year onwards, i.e. CAT 2019 onwards (once you are in your final year or complete graduation). However, it will be ideal to start preparation from now if you want to get a headstart against your peers. Take  a look at this article, which I'm sure will guide you about when and how to start preparations for next year - 

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You may also take  a  look at the accounts of toppers in CAT Topper interview.

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