I am having severe pain in chest and upper back for past 1 week. I lifted 6 kg weight for fun and this all started after that...how to cure this?

Weight lifting. You apparently strained your chest,back and neck muscles when you tried to weight lift.That can happen when you are out of shape.
Might take time. muscle sprains can take time to heal, use NSAIDS regularly, unless contraindicated, for a week, if no improvement see your doctor, good luck.