The first step in urine formation is glomerular filtration, where blood plasma is filtered through the glomerular capillaries into Bowman's capsule. This process creates an ultrafiltrate free of cells and large proteins but containing water, electrolytes, and small molecules. Glomerular filtration sets the stage for subsequent urine processing, including reabsorption and secretion in the renal tubules. Without this initial filtration, no filtrate would be formed for the kidneys to modify. Hence, it is the primary event initiating urine production.

Reference: Guyton and Hall, Textbook of Medical Physiology, 13th Edition.