Fuel requirements:
Conventional rockets accelerate by ejecting high speed propellant. As the propellant blasts out of the rocket in one direction, it pushes the spacecraft in the other - Newton's third law. We need a highly efficient energy source that can provide propulsion for a minimum mass of fuel.
One possibility: the mutual annihilation of matter and anti-matter (matter into energy according to E=mc2).
Example: suppose we want to reach a speed of 99% the speed of light:
Ideally we need a propulsion system that needs no propellant.