hello,on singing when should we use the head voice and when should we use the chest voice?

Hi, Mouna! In general, chest voice should be used in the lower portion of your voice, and head voice in the higher portion. Most female-identifying singers have a natural "passaggio" or point of transition between chest and head register somewhere between Eb4 and G4. You can sing higher than that in chest voice, and you can technically sing below that in head voice - but your voice would naturally use chest function for lower notes (below Eb4-G4) and head function for higher ones (above Eb4-G4). 

We train mixed voice within this middle section of our voice to help disguise that register shift, and so that we can sing higher pitches powerfully, but safely.