Does tank keyword only work when there is a combat? Or if opponent have 2 units at a BF and I want to play Falling Comet, do I need to target tank first?

The Tank keyword only functions during the Combat Damage step of a showdown. It has no effect on spells, abilities, or any other game actions outside of combat.

According to Rule 815.1.b, Tank is functionally short for: "I must be assigned lethal damage before any other unit with the same controller as me that does not have [Tank] during the Combat Damage step."

Regarding your question about Falling Comet:

1. Tank does not apply: Because Falling Comet is a spell and not combat damage, the Tank keyword is ignored.
2. Targeting: Falling Comet says "Deal 6 to a unit at a battlefield." This targets a single unit. You are free to target any unit at the battlefield, regardless of whether it has the Tank keyword or not. You are not required to target a unit with Tank first when using spells.

As confirmed by FAQ 7743: "Tank only applies to combat damage and does not affect spell damage."