Although there is no set date for the beginning of the Renaissance—like in 1401 everybody referred to things of the previous year as "so Medieval!"…nuh uh—I'd place it close after the perfection of the printing press by Johannes Gutenberg in the mid 15th century, which made books widely available. It was the invention of a millennium.
The painter who made much of the transition from old to new possible was Fra Angelico. Here is his Annunciation fresco (the Annunciation is the moment when the archangel tells Mary she is to give birth to God's Son).
We see characters moving in space again, and the space is defined using linear perspective (which I'll get to in just a moment). Notice the columns? The front ones are Corinthian and the ones on the left are Ionic. This is the just the beginning of what will be the return to Greco-Roman ideas and art styles.