I am having a tournament in college which gets me to taluk level if i win. This is the most important point of my life for my chess career! I am a 1500 rated player in standard 15 10 game. The tournament timings is of 20 mins per player. I want you to help me by saying which defence should i adopt to play against a those 1600 players against e4 and d4. please help me!!

 Right now i am playing d6 for both e4 and d4.  just like this:
 

play the openings that you already play. trying new openings that you never played before will not bode well in a tournament.

Calm down. You will have a life time of opportunities to do well in chess.

Asking what defense to play is too broad a question. It depends on what they open.  What do you play now and how well do you know the openings?

Saying you should play say the Sicilian Defense means nothing as you have not given us enough information as to what you have learned already.

I use a combination of A41-Rat Defense and C41-Philidor Defense: Lion Variation. The move order is the key for deciding how to combine them.

It depends upon what your opponent does. The basis behind the idea is to obtain the same basic objectives as other more popular e4 and d4 defenses, such as,  a chance to fight for central control and the ability to coordinate your pieces for both attacking and defense. Also the idea allows you the luxury of not being forced out of the position that you wanted to use. It is more flexible that many defenses. You can use it against e4,d4, and c4. That gives you far more practice. Usually you have to change defenses for each opening and it takes longer to get good at defense. Also you can avoid many of the common opening traps that result from playing all of those other defenses.

Be warned, this position is somewhat crowded. It is a positional defense. It is not an open position that allows for easy tactical chances early on, unless your opponent plays horribly. I recommend studyin g how an engine plays against this and how it responds against it, if possible. I also recommend practicing this as many times as you can before you play that tournament against human opponents. I will try to get a link to a Fide CM who teaches his students this same basic idea.

http://www.chess.com/forum/view/chess-openings/rat-defense-philidor-defense-lion-variation-transposition

Check out this forum. The Fide CM posted in this forum. I posted a few games there one against e4 and one against c4. IM Pfren gave some thoughts about how to play this position too. Another player showed one of the problems with it and if you study its weakness you can learn to avoid this or counter it with best play. I am sure that it will force your opponents to calculate and not play from memory. It can instantly make your chances more equal if not give you an advantage. Good Luck.