This is fairly simple question, but I'm unsure. Suppose I use enchant my basic land with an aura like Caustic Tar. The enchantment's ability says to tap the land. Can I tap it immediately to use the ability, or do I have to wait till my next turn? I thought you could tap it, but my opponent didn't think it was valid.
Yes, you can immediately tap it to activate the ability.
Your opponent was perhaps thinking of "summoning sickness", which prevents creatures from attacking or using abilities that have the tap or untap symbol in their cost. But that only applies to creatures, not lands - just as you can tap your Swamp for mana the turn you play it, you can tap it for another ability immediately if it somehow gets one, like from Caustic Tar.
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.