when i press record, the song also got recorded instead of just recording my voice, & when i played the song back my voice sounded alright and the song sounded like it was recorded from a camera 
What song? Where is that playing from? We can’t see what you are doing.
i was playing an mp3 format, and the song is playing from audacity
Do you mean that it is an MP3 file that you have Imported into Audacity?
If so, ensure that “Transport > Software Playthrough” is disabled (not selected) and ensure that the “Recording Device” selected in the Device Toolbar is set to your microphone and NOT to “Stereo Mix”.
i have done all of that but it still doesn’t work 
I still can’t see your computer 
Please describe your set-up in detail and describe what you are doing and what is happening.
i’m just 13 years old and i don’t know what else to explain
so my micro-phone is a built-in input, and its 1 (mono) input channel
the last time we started recording we didn’t have this problem
we didn’t get the song to get it recorded whilst we were singing and now it just happened
and what is an overdub?
Overdub is when you have one (or more) track(s) in Audacity and you record another track into the same project.
How are you listening to the backing track?
You need to use headphones otherwise the microphone will pick up what is playing through the speakers.