I am really sorry if this post is so shot and newbie but I will use these instructions on my next posts.
#> sudo -i
#> cd /usr/src
#> apt-get source linux-image-$(uname -r)
If fails complaining about dpkg-source, you need the debian package development tools.
Install it doing:
#> apt-get install dpkg-dev
and try to download the source again.
You can get using git. If you prefer this method check this page.
I am using root (sudo -i) on this example because next post I will compile some code that requires the right permission to run the example codes.