how can I take a good indoor picture of a christmas tree with the lights on?
Further to Dr Sam’s answer, do everything he says and then also play with lighting, bring other lights / lamps into the room and try to balance the lights shining from the tree with the light reflecting from the tree, so that you see an equal amount of tree and lights in your photo. Too much light (eg flash) will leave you with a bright tree with insignificant lights. Too little light will leave an array of bright lights, with black in between them where the tree should be. Get creative and have fun!