As a note, we're getting all of this information from Mark's written logs.
Mark feels a bit better the next day. Based on his calculations, he has enough food to last over three hundred days. That's not too bad. He's also got some spare spacesuits and two rovers, though the MDV was trashed during the storm.
He also runs a diagnostic of the Hab's systems. The oxygenator is great. The water reclaimer is great. Even the solar panels are in perfect condition, though he'll need to "sweep them off every few days" (2.11). All in all, it could be a lot worse.
Mark has two specialties—mechanical engineering and botany. That first one will come in quite useful should anything break down. The second sounds pretty useless (grow anything on Mars? come on) but you never know...
All in all, Mark's first thought for survival is to wait for the Ares 4 crew. There are two problems with this, however. One—the Ares 4 landing site is "3200 kilometers away" from his Hab (2.19). Two—they won't arrive for four years.
Basically, he's be deader than a doornail before then.
A few days later, he tries to locate the communication dish that got blown away during the storm. No luck.
Then, Mark gets struck by a bolt of inspiration—he's a botanist, for Pete's sake, so why doesn't he grow some plants? After all, he has seeds and some earth soil, and though no one else has been able to grow plants on Mars yet, nobody's life has depended on it yet.
He's also got a steady stream of fertilizer—his butt. (Gross.)
Basically, he'll make a cocktail of earth and Martian soil, adding a bit of his own poop for good measure. Mark's hope is that the earth soil will eventually fertilize the dead Martian soil, greatly increasing his amount of available farmland.
And where exactly will he plant this garden? The Hab, of course! Two-thirds of the building will be turned into a farm, where Mark will grow potatoes because "they grow prolifically and have a reasonable caloric content" (2.69).
It won't solve all his problems, but it's a good first step. Plus, gardening's kind of fun.