Dead battery

Okay, about two months ago the lights would not turn off. The battery died of course. This car I'm not familiar with. I bought it from individual and he's not mechanic minded at all and knows nothing when asked a question. But the car is immaculate and I stole it. So anyway it will start but when the blinker is turned on or the lights it shuts off.
Does anyone know what could cause of this issue? Thank you. JC