This is kind of like asking an artist which he paints first...the tree or the water. It depends on which one strikes his fancy!

Same with songwriting. I may be playing a few chords on my guitar and some lyrics begin to fall into place. Other times, I may be thinking of a situation, say...like Mike said above...the harvest. That may lead to a line or two of lyrics...

The corn is in the crib and the hay is in the barn; The wood is neatly stacked to keep the family warm...

With those lyrics, I pick up my guitar and try to find a rhythm that feels right - using a simple I, IV, V chord progression. It's funny but most times it falls into place pretty easily.

After I get something that seems right, I'll go to the keyboard and try to fill in the rest of the song...

Hope this helps.

Good Luck Jim