Start from a new file in Blender. Depending on your start-up file you might already have a light in your scene. Press F12 to render. If you get an error about no lights in the scene then simply add an area light (Shift A->Lamp->Area) and press F12. (image of first render)
Lighting a scene is usually done by area lights or a HDRI. Go to the World panel and enable the HDRI option and open a HDRI file. Render.
If your start-up file in Blender has the sun lamp, you might not see the HDRI light up your scene now because the sun lamp is very bright and overpowers the HDRI. Stop the render (ESC) and enable viewport rendering (Shift Z). Select the sun lamp and decrease its Gain to something like 0.00001. Yes, the sun lamp is bright :) Our real sun is bright too and LuxCoreRender is based on real world physics.