Vauxhall Cavalier battery dying

Starts sometimes in mornings. Soon as I turn key to heating car ill know if car will start is abs light is bright and like wise if abs light is dull it won't. Driving tonight dash lights dimmed slightly and abs light came on. Drove home and tried to start but battery dead. Turn ignition on and lights are dim at 1st until you hear something like a relay clicking and dash lights brighten but battery still dead. Any help?