1996 Chevy Monte Carlo wont start

My monte carlo wouldnt start my wife went through the gears and it finnally started. Today it wouldnt start. Engine doesnt crank no power. Lights dim checked battery voltage 13.42. Clock on the radio has to be reset as if I disconnected the battery. I noticed when going through the gears the dash light seem dim and no lights in reverse. Tried jump starting the engine seemed needed more juice almost wanted to crank not enough power. Please help me.