Hello Rajpal,

Graduate aptitude test in engineering(GATE) is an examination which tests the understanding of subjects in engineering and science.It is always held in the first two weeks of February. It is one of the toughest exams in india. Gate score is used to get into various post graduate education programmes as well as into several indian public sector units.  

     The first step of starting preparation for any competitive exam is by knowing the syllabus thoroughly and knowing the weightage of each subject. Firstly, u refer the previous papers and detailed gate advertisement listing the subjects which are covered in the exam paper. In the detailed advertisement, u can also see the sub topics from where the questions are given. So u have to focus on that sub topics only. Here am listing out the subjects and their approximate weightages, so that u can knw which subjects are to be stressed upon.

1. General aptitude-15

2.engineering mathematics-13 to 15

3.manufacturing engineering-9 to 19

4.thermodynamics-8 to 12

5.fluid mechanics-6 to 11

6.strength of material-6 to 14

7.theory of machines&vibration - 7 to 15

8.industrial engineering- 4 to 9

9.machine design-2 to 7

10.engineering mechanics-2 to 8

So, the first five subjects carry almost 50-60 marks so studying these subject to get more score in GATE.Since you are in your first year, all you can do is follow these lectures very carefully, have a look at nptel videos, and try to practise aptitude as much as you can. And for competitive exams like gate accuracy is more important. So more the practice you do, the more accuracy u get. 

          And yeah, since you want to get into DRDO. Here is some information u need to know about DRDO.

Defence research and development organization (drdo) is one of the nation's leading research and development Institute that recruits scientists in group 'A' gazetted technical service. The salary is  around Rs 60000/- per month.

Eligibility:

1. A candidate should be an indian national.

2. A candidate must have scored minimum 60% marks or equivalent 6.75 cgpa in the qualifying exam

3.maximum age of the  candidate must be 28 years.

4. Candidate having knowledge of german,french,russian, Japanese or chinese is desirable. But not compulsory.

 Before applying make sure u meet all these criteria. Prepare well!! I hope u good luck for ur future.

Hi Rajpal
 
Before starting off with any studies, it is important for you to know the complete syllabus. You should check the official syllabus of Mechanical Engineering (ME). Also, the earlier you start your preparation, the better it is. This will give you time to study all of the topics properly with additional time for revision. 
 
You should solve sample papers and mock test during the preparation process. This will allow you to practically apply what you have studied and check your mistakes so that you can rectify it.
 
To know how to prepare for GATE 2019 Mechanical Engineering (ME) in detail, check out this article- https://engineering.careers360.com/articles/how-prepare-for-gate-mechanical-engineering 
 
Hope this helps.