How would the human race build a starship?

You are right, it would take more resources than the entire Earth could afford to ferry sufficient materials into Earth orbit and build a starship weighing several million tons. And if say, a 30 million ton starship were to take off from the Earth's surface it would have to be propelled by nuclear energy, chemical energy would simply weigh too much and Earth's stocks would be seriously depleted. Such releases of energy would have a devastating effect on ground surface and local weather conditions. The vertical thrust, a continual release of energy equivalent to a 30 megaton hydrogen bomb would be nowhere enough to put it into orbit. It all reminds me of the big talk by some scientists, that certain planets in our solar system could be converted into places of human habitation by modifying the atmosphere, releasing the frozen water etc. Again, the cost, energy and logistics would be simply impossible.