Mars has a number of characteristics that make it a feasible option for colonisation, the most important in my opinion being the presence of water, which is in ice form due to average surface temperatures significantly below zero. Oxygen can then be extracted from this water using a power supply such as solar or nuclear energy. The living quarters can be set up prior to the arrival of the settlers, and the extraction of water and oxygen monitored. As long as the equipment necessary for living on Mars can be brought and replenished from Earth, with these three essential ingredients available, namely energy, water and oxygen, I believe both humans and the plants they will eat can live on Mars. How people will deal with life on Mars psychologically, however, is another question!
We will rely heavily on technology for our survival- on Mars the atmosphere is 96% carbon dioxide, the pressure 0.6% of sealevel pressure on Earth, the temperature -63 degrees Celsius on average and without a protective atmosphere the solar radiation is extreme. Humans would die fairly quickly on the surface of Mars without technology!
In spite of these challenges, in fact Mars is the obvious destination for the next Great Trek! Humans are explorers by nature and now for the first time the possibility to colonise another planet has arisen. I feel like this is something I am meant to do rather than want to do. I feel like my whole life has been leading up to taking part in this awe-inspiring mission to call another planet home. Over 95% of species ever to have existed on Earth have eventually become extinct. By creating a home for terrestrial life on another planet, we have an insurance policy against disasters which may be self-inflicted or the result of such events as large asteroid impacts.