It's worth checking what people have already calculated, e.g. the wiki article on the topic [0]. There's large amounts of CO2 ice on Mars which could be "evaporated" to create a strong greenhouse effect. Someone thought of orbital space mirrors as an elegant method to do this -- to heat the polar ice caps and sublimate enough to create a runaway greenhouse. Fluorocarbons would take about 10^8 tons, so that might be vaguely plausible.
Does Mars have an atmosphere to insert the gas into? How much mass does a planet need to support an atmosphere? A quick check on Wolfram suggests that Earth has 10 times the mass of Mars. Does a planet need to be Earth or Venus sized to hold onto the air?
Mars does have an atmosphere, but it's thin. As for holding onto the air, it all depends on what kind of timeframe you're talking about. The Moon, for example, has no atmosphere because it's too small. But if you magicked into an existence an Earth-like atmosphere on the Moon, it would persist for something like a million years before it all drifted away. Mars would do even better. That kind of timescale is probably plenty for human purposes.
Earth's retention of its atmosphere also owes a significant amount to its planetary magnetic field, which deflects solar radiation that would otherwise energize molecules in the upper atmosphere enough to escape the gravity well.
Add the biggest amount of C and H you can on a molecule with it remaining a gas (easier on Mars than on Earth). Then, replace some of them with N and O, in a way that you create the biggest diversity of shapes you can.
