How can i start preparing for GATE exam ?Now i am in 3rd sem BE(CSE) .

Dear aspirant,
Here are some tips for preparing for GATE exam for Computer science engineering.
Analyse the syllabus properly  , first look for what is there in the syllabus which is necessary for you to prepare for the exam .
Manage time properly  : You need to manage time , as you have to attend college and prepare for your end semester examination plus your vivas that happen during your semester and even your mini projects which you will do in your 3rd year. So time management is the key . You can't do all the stuff together , so manage time properly and wisely.
Understand the topics first. Before solving any question in GATE , you gave to understand every minute detail in your syllabus.  If you can't understand a topic , you can even ask your professors about it.
Make short notes which will make easy for you to revise  .
Solve different question papers. Solve at least upto 5 years ago  question papers for GATE as it will give you an overall idea about what trend is going on .
If you have any friends who want to give GATE exam then study together , as it will give you more motivation for this examination.

THE MAIN IMPORTANT POINT FOR YOU IS THAT YOU SHOULD JOIN AN COACHING CLASS FOR GATE. THERE ARE MANY COACHING CLASS WHICH WILL GIVE YOU OVERALL IDEA ABOUT HOW TO SOLVE THE PAPER EFFECTIVELY AND WISELY.

All the best.

Hello urvish Patel,
Now you are in 3rd semester
Computer science
So firstly focus on your all subjects in your semesters of all years (first year maths includes)
Focus on mainly all core subjects once you are interested and perfect in your semesters subjects thats enough you can do it well in GATE examination
All the best