Aquinas' Third Way
The Third Way introduces two crucial concepts: contingent beings (things that depend on something else to exist) and necessary beings (things that exist independently and don't need anything else).
Everything you see around you is contingent. You exist because your parents existed, trees need soil and water, planets need gravity. These things are temporally dependent - they rely on outside factors both to come into existence and to keep existing.
If everything in the universe is contingent, then something necessary must have brought them all into existence. This necessary being - independent, uncaused, and eternal - is what Aquinas calls God.
Here's Aquinas' example: wood has the potential to be hot, but it needs fire (something already actual) to make it happen. God is the ultimate actual being that makes everything else possible.
Study Tip: All three ways use inductive reasoning based on a posteriori premises - they start with observations about the world and work towards conclusions about God.