To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases (such as that of Tithe Taker's effect), then apply any cost reductions. The mana value of the spell remains unchanged, no matter what the total cost to cast it was.

An opponent's activated ability that costs no mana to activate will cost {1} plus its non-mana costs during your turn.

Because blockers are chosen all at once, you can't block with a creature with afterlife, wait for it to die, then block with the resulting Spirit tokens.

Activated abilities contain a colon. They're generally written "[Cost]: [Effect]." Some keyword abilities are activated abilities and will have colons in their reminder text. An activated mana ability is one that produces mana as it resolves, not one that costs mana to activate. Triggered abilities (starting with "when," "whenever," or "at") are unaffected by Tithe Taker.