Can I use an ability granted to a land by an aura immediately?
Yes, you can immediately activate the new ability by tapping the land. The enchantment gives the land a new ability, and that ability can be activated just like any of the land's other abilities. Of course, you can only do this if the land is untapped, like any other ability with a tap symbol in the cost.

The one exception is if the land is a creature and it just entered play that turn (for example, if it's a Dryad Arbor or it was animated by Nissa, Worldwaker). In that case, like any creature, it has summoning sickness and can't activate abilities with a tap symbol in the cost until your next turn.