How do i configure my favorite song to start when i boot my Debian PC?

The answer depends a bit on what desktop you're running, because they have different startup procedures. KDE has a "system notifications" section in its control center that can be configured to run sounds on events, for example.

But in general, all you really have to do is set up a command and have it run on login. The problem is that there are many different ways to do this. I would personally just create a shortcut or script with the command "play mysongfile.ogg" ("play" is provided by the sox program, you can use other commands like mplayer instead) in your DE's startup/autorun folder or in some startup file.

I'm using Debian Lenny amd64, and i have Rhythmbox and Totem player...So, how do i implement what u suggested?

You could just go into sound events..and choose the song as log on sound..

You could probably add a script to /etc/rc2.d. There's a README file so follow the naming convention to get your script to run. You will probably need a high number like S99xxxx so that it tries to run late in the boot cycle. Your script will probably look something like this:

/etc/rc2.d/S99bootsong

You could just add the second line to /etc/rc.local and dispense with creating an extra script. /etc/rc.local is normally the last script to execute on a normal boot. (It doesn't run at all when you boot into single user mode, aka recovery mode.) If you decide to add this to /etc/rc.local, make sure you add it before the exit 0 zero statement! (Yes, I made that blunder once.)

Note that whether as a separate script or as part of /etc/rc.local, this will run when the system boots while some of the earlier suggestions run when you log in.