A landfall ability triggers whenever a land you control enters for any reason. It triggers whenever you play a land, as well as whenever a spell or ability puts a land onto the battlefield under your control.

A landfall ability doesn't trigger if a permanent already on the battlefield becomes a land.

If the creature spell has {X} in its mana cost, use the value chosen for X when calculating that spell's mana value.

Whenever a land you control enters, each landfall ability of permanents you control will trigger. You can put them on the stack in any order. The last ability you put on the stack will be the first one to resolve (as a result, you can have those abilities resolve in the order of your choosing).