The f of Spanish is pronounced basically like the usual "f" sound of English, although perhaps a bit softer. Thus it is similar to the "f" sound of "fast" and "friend," but never the "v" sound used in "of."
The Spanish f sound is classified as an unvoiced labiodental fricative, which is a way of saying that it is pronounced by touching the upper teeth to the lower lip and without using the voice. It almost always appears at the beginning of a word (as in fiesta) or after a prefix (as in reformar or bifurcado).
Words spoken by native speakers in the accompanying audio lesson on pronouncing the f are fogón, fijaron, francesas and franceses.