[P.O.E.]

First send some SUVs to generate CO2, then send up some plant seeds and miracle-gro.

[MHGinTN]

The gravity of Mars is not sufficient to hold a 'light' atmosphere, so Mars needs water since water vapor is the heavier of the atmospheric gases, much more so than CO₂. Find a few asteroids and/or comets of size which are mostly water and direct them to the Martian surface ... that's if there are no living creatures on Mars. There is a Moon of Saturn or Jupiter (IIRC) that is a water orb, so bust a few trillion gallons off that orb and take them to Mars.