The cost to activate a creature's outlast ability includes the tap symbol ({T}). A creature's outlast ability can't be activated unless that creature has been under your control continuously since the beginning of your turn.

Several creatures with outlast also grant an ability to creatures you control with +1/+1 counters on them, including themselves. These counters could come from an outlast ability, but any +1/+1 counter on the creature will count.