A land that becomes a creature may be affected by "summoning sickness." You can't attack with it or use any of its abilities (including its mana abilities) unless it began your most recent turn on the battlefield under your control. Note that summoning sickness cares about when that permanent came under your control, not when it became a creature.

When a land becomes a creature, that doesn't count as having a creature enter. The permanent was already on the battlefield; it only changed its types. Abilities that trigger whenever a creature enters won't trigger.

Each time you activate Raging Ravine's last ability, it gains an instance of the triggered ability. For example, if you activate the last ability twice and then attack with Raging Ravine, both of the triggered abilities it gained will trigger. It will get a total of two +1/+1 counters.

Any +1/+1 counters put on Raging Ravine remain on it even after it stops being a creature. They'll have no effect until it becomes a creature again.