how to get source code

Apt-get can pull down source debs of packages in the repos

Which bugs?

I suggest you start here:
https://wiki.ubuntu.com/HelpingWithBugs

That page provides a lot of information and useful links on how to triage bugs, where to get source code etc.

Each package as a page dedicated to bugs on Launchpad see here for example:
 https://launchpad.net/ubuntu/+source/hello/+bugs.

and you can find the source for that package here

 https://launchpad.net/ubuntu/+source/hello

The bugs which we want to fix are
 #935296 java-atk-wrapper prevents java applications from starting
 #948972 compiz segfaults when mounting/unmounting a loopback device quickly
 #966913 Akonadi contact resource wrong defaults
Give us the package name for these bugs so that we can get the source code

Bug #935296
https://bugs.launchpad.net/ubuntu/+source/openjdk-6/+bug/935296
Package: openjdk-6-jre 6b24-1.11.1-0ubuntu1

Bug #948972
https://bugs.launchpad.net/ubuntu/+source/gnome-disk-utility/+bug/948972
Not sure which package yet

Bug #966913
https://bugs.launchpad.net/ubuntu/+source/kdepim/+bug/966913
Package kdepim

Two of these bugs are already fixed in Debian so you may want to start by looking at what the fixes are they may apply directly to Ubuntu and just need incorporating. This is not guaranteed however the fix may be different for Ubuntu

Good luck.

Thank you

Please get me the package name of this bug
#948972 compiz segfaults when mounting/unmounting a loopback device quickly

^It's either compiz or gnome-disk-utility.

Packaging branch status: OUT-OF-DATE this error i am getting for the package kdepim.Bug is #966913 Akonadi contact resource wrong defaults.So how to get the source code please help me

Kdepim seems to be a good example for understanding the "upstream" concept.
http://www.freedesktop.org/wiki/Distributions/Packaging/WhyUpstream/

If bug #966913 is caused by a weakness in kde's original source, the solution to the problem must start at the kde source and will sooner or later replicate to Ubuntu. If the bug, however, is caused by deviations from the upstream original by Ubuntu (e.g. for packaging purposes or better integration in Ubuntu), then the solution has to be done in the modifications that Ubuntu is responsible for.

If I look at bug #966913 I see a link to kde-bugs #296944
So I guess that the problem is/was in KDE's original source. The KDE bug is already marked as 'resolved fixed'

So in this case it seems to me that the required step to bring that into Ubuntu is to perform a sync of the Ubuntu package with the KDE original source.