Hello lachit, Good to hear, you have interest in defence sector...

TIps for written exam - 1) Should grip on math and English

2) Learn to manage time

3) Focus on basic concepts

4) Your understanding should be clear

5) Solving multiple question papers, this will help you to prepare for all kinds of questions

6) Previous papers should be attempted to understanding question patterns

7) Be confident and calm

For Running - You can go through Youtube web videos to see what their requirements are and what all type of tests they take which are necessary of qualifying defence exam...

For Eligibility you can go through this link - http://www.ssbcrack.com/2012/06/how-to-join-indian-armed-forces-after.html

I hope, i answer all your question... gud luck