This is a triggered ability, not an activated ability. It doesn't allow you to tap Night Market Lookout whenever you want; rather, you need some other way of tapping it, such as by attacking.

For the ability to trigger, Night Market Lookout has to actually change from untapped to tapped. If an effect attempts to tap it, but it was already tapped at the time, this ability won't trigger.

In a Two-Headed Giant game, Night Market Lookout's ability causes the opposing team to lose a total of 2 life.