The word yirah is typically translated as “fear” but that carries all sorts of shades of meaning that are unintended. For example, in Mitzvah #212, we were told to “fear” our parents. Since our parents are (presumably) not slashers from a horror movie, we rendered yirah there “deference.” Same here. God is not out to get us, so fear does not mean terror. Rather, it means awe or reverence.
The best thing is to do mitzvos from a love of God. An awareness that He loves us and gave us mitzvos in order to benefit us should motivate us to do His will. Going against His will should be unthinkable from our deep love of Him. Nevertheless, human nature being what it is, a healthy “fear” can be an effective deterrent. Being in awe of God can help keep us from straying off the proper path.
This mitzvah applies to men and women in all times and places. It is discussed in the Talmud in tractate Sanhedrin (56a). This mitzvah is codified in the Mishneh Torah in the second chapter of Hilchos Yesodei HaTorah. It is #4 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos and #4 of the 77 positive mitzvos that can be fulfilled today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.