“There are two ways to spread happiness; either be the light who shines it or be the mirror who reflects it.”
Edith Wharton

It’s, of course, best to be the light who shines, but if you can’t do that for some reasons, you can at least reflect it.
The easiest way to spread happiness is to simply smile. To reach another level, say hello. If you are feeling bold, you might even help someone.

It’s not that tough, try at least once.