You cannot attack or block with a creature that is tapped. So you are correct, if you tap a creature to use an ability before the attack phase, you won't be able to choose it as an attacker, and it won't be an available defender during your opponent's next turn.

Do tapped blocking creatures deal damage?

TAPPED CREATURES DEAL NO DAMAGE. Change: Under Fifth Edition rules, tapped blocking creatures dealt no damage in combat. Under Sixth Edition rules, they do.

Do cards tap when blocking?

Yes. Blockers only need to be untapped when they're actually declared as blockers; after that, tapping & untapping have no effect on blocking or any other parts of combat.