I am in b.tech 1st year should I start prepartion for CAT HOW .

It's good you want to start early.
Now first things first, let's talk about things other than CAT.
1. Score as high as you can in your exams for Engineering. It matters, a lot.
2. Start building a good profile by doing relevant projects and internships throughout colleges. Don't sit idle.

Now, as for CAT.
1. Join a coaching institution.
2. If you can't join an institution, Google for free resources and guidance, you'll find plenty for free.
3. Stick to a plan.

See, you have plenty of time. But that's where we slack, and waste the time we have. Don't do that, start right away by picking up a basic book for Aptitude and Reasoning, and start practicing.