Zar Ojanen, Scion of Efrava's ability is a triggered ability, not an activated ability. It doesn't allow you to tap it whenever you want; rather, you need some other way of tapping it, such as by attacking.

If Zar Ojanen becomes tapped while casting a spell or activating an ability, that spell or ability resolves after Zar Ojanen's triggered ability has resolved.

Domain abilities count the number of basic land types among lands you control, not how many lands you control or how many of any type.

The basic land types are Plains, Island, Swamp, Mountain, and Forest. Land types other than basic land types (such as Desert) don't contribute to domain abilities.