Which ESC should i get? 20A , 30A or 40A? what does that depend on?  

Please mention about your motor so that we can suggest you the required ESC.

Sai Krishna
www.rcbharat.com

umm i havnt yet started. all mumbai closed till tomorrow. so i'll be starting from tuesday. i want to build a x quadcoptor so suggest me which motors should i get and ESCs. these cost the most so i want to consult before i buy

Please don't name a topic like "help help help".  Put a meaningful topic name, like "Which ESC should I pick" or something like that.

[Admin Note] Topic name changed to a meaningful one.

Regarding how to pick the right ESC/motor/prop combo, please read the relevant threads in the "Gems" list (especially the ones with topic names "Understanding electric power systems" and "Amp rating and motor size").

http://www.rcindia.org/rc-india-forum/the-rc-india-gems/ 

Hi Dhruv,
Hope that the below link will clarify your doubts regarding the selection of the motors esc and props for your quadcopter.

http://thequadcopterguy.blogspot.in/p/choosing-your-parts.html

Thanks
Vivek Kumar Singh

a quick question. What if i use 45amp ESC for 30 amp motor?

A 45 A ESC for a 30 A motor sounds fine! Which motor did you finally make up your mind for? 

Quote from: vksingh on November 19, 2012, 10:19:26 AM

http://thequadcopterguy.blogspot.in/p/choosing-your-parts.html

OMG this is completely copy pasted material of my thread. You can see that buddy forgot to delete some lines which clearly states that it's copy pasted from RCindia.

Lets get down to understanding what goes into selection of the ESC .
Q : What does the ESC do ?
A : It controls the motor's , speed hence the current draw .

Hence all you have to do is figure out how much maximum current will your motors draw  and you have the answer to what ESC you should use , higher than the max current draw : pretty simple and straight forward right 
The selection of ESC is important because the BEC in most ESC's supplies power to the receiver hence should the ESC or BEC  burn out then the end result is pretty much a foregone conclusion  .

Sai