Parts you need:
1 Power source of 12v;
1 Transistor (in this case we use the BC547);
1 Resistor of 1KOhm;
1 Resistor of 680KOhm;
1 LED (in this case blue);
Some condutor cables to connect the parts.
Step 1: Connecting parts in the breadboard
Step 2: Connect everything
Remember, the LDR resistance depends on the light you have on the room or where you are constructing that. If you have light, the LDR resistance is low so, the LED should be turned off. When you obstruct the light that focus on the LDR, the LED should light up.
If for any reason your LED is always lighted up, try to chance the transistor. Switch the Emissor and the Collector. If the problem persist, increase the resistor you have in the base.
Step 3: All should be working
Try your circuit and see what happens.
Here's the video of my circuit working:
If you have any questions feel free to ask in the coments. If you think I should improve something, let me know. It's my first guide so it's normal I made some mistakes.