The three parts of the ability happen sequentially. For example, if you control a creature with 9 power when the ability starts to resolve, you'll put a +1/+1 counter on each creature you control — and now, since you control a creature with 10 power, you'll gain 10 life.

This ability triggers at the beginning of each of your upkeeps. Whether you control any applicable creatures is checked only when the ability resolves.