Building cities in space will require materials, energy, transportation, communications, life support, and radiation protection. Launching materials from Earth is very expensive, so bulk materials should come from the Moon or near-Earth asteroids where gravitational forces are much less, there is no atmostphere, and there is no biosphere to damage. Our Moon has large amounts of oxygen and metals, but little hydrogen, carbon, or nitrogen. Near-Earth asteroids contain substantial amounts of metals, oxygen, hydrogen and carbon. Asteroids contain some nitrogen, but not necessarily enough to avoid major supplies from Earth Energy.
Solar energy is abundant, reliable and is commonly used to power satellites today.
Massive structures will be needed to convert sunlight into large amounts of electrical power for settlement use. Energy may be an export item for space settlement, using microwave beams to send power to Earth. This is the key to any space endevour. To settle space we need much better launch vehicles and must avoid serious damage to the atmosphere from the thousands, perhaps millions, of launches required. Transportation for millions of tons of materials from the Moon and asteroids to settlement construction sites is also necessary.