The triggered ability results in three sequential events: you gain some life (subject to effects that might alter life gain), then lose some life (subject to other effects that might alter life loss), and then put the card into your hand.

If a creature card has "*" in its power or toughness, the ability that defines "*" works in all zones. Note that the card is in your library when Sapling of Colfenor's ability checks the card's power and toughness.

If the revealed card is not a creature card, it remains on top of your library.

Lethal damage and effects that say "destroy" won't cause Sapling of Colfenor to be put into the graveyard. However, Sapling of Colfenor could be put into the graveyard if its toughness becomes 0 or less, if it's sacrificed, or if you control another Sapling of Colfenor (due to the "legend rule").