Christmas-lights starburst effect

How can I get that starburst effect when shooting Christmastree light?

Stop down to a smaller aperture (smaller than f/8 or so). The more you stop down the more pronounced the effect will be. However, below about f/11 to f/16 you'll start losing sharpness due to diffraction.

Alternately, you can buy a Starburst filter for your lens. That will give you more control over the # of rays on each light and can be used with larger apertures.