The problems faced in establishing some form of sustainable colony on Mars:
1. Radiation. Mars has no significant magnetic field, and solar radiation already has stripped much of the Martian atmosphere, leaving only a tenuous (1/100 the atmospheric pressure of Earth) veil consisting almost entirely of carbon dioxide, with is about 1.4 times as heavy as the nitrogen-oxygen atmosphere of our planet. Earth's magnetic field deflects most charged particles emitted from the Sun. Not any protection like that on Mars.
2. Water. Mars has some water, that is certain. But not much anymore. That too probably has been stripped via solar radiation and the effect of weak gravity; water vapor is much lighter than our nitrogen/oxygen mix.
3. Energy. Mars gets about 1/4 the intensity of solar energy that the Earth gets. Solar energy was just barely enough to keep the two wonderful small solar rovers, Spirit and Opportunity, roving. The bigger thing, Curiosity, runs on a nuclear engine. It's about te size of a Volkswagen Beetle. There exists no fossil fuel resources on Mars. Energy-generating technology will need to be shipped there from Earth.
4. Construction materials. Likewise, not really available locally on Mars. Yeah, there's a lot of iron oxide, so, theoretically, you could make iron. But that takes a heap of energy. See No. 3, above.
5. Food. The concept of growing crops on Mars is fraught with difficulty, including both the radiation and weaker sunlight issues. The necessity of water might be overcome, but also there's the need for any plant crop to have organic material, and things like phosphate and nitrate fertilizers, to grow.
6. Glass. If you're going to establish a colony on Mars where plant crops can be grown, among other things, you need transparent biosphere kinds of places, which would require glass. Glass is heavy. The idea of transporting glass material for construction of such things on Mars is, frankly, ridiculous, purely on the basis of the energy cost. And on Mars there exists no free silica for the manufacture of glass, as far as we can tell. Free silica exists on Earth only because of the surficial chemical refinement via plate tectonics that has been going on for four billion years. Mars has no such plate tectonic history which would generate free silica.
In addition to which, the manufacture of glass, even with free silica available, is yet another immense energy problem.
7. Return. Given the absence of energy resources on Mars, any possible return from the Martian surface would require that the energy resource necessary to launch be sent to Mars from Earth. The initial energy cost of such a thing is, literally, astronomical. We had a hell of a time sending a Volkswagen-sized vehicle to Mars to explore the surface. The quantity of fuel needed to re-launch such a thing back into space on a return voyage to Earth is immense. This alone is why we've never even tried to design a robotic mission to retrieve surficial material from Mars. Not to mention the issue of actually getting a relaunchable vehicle safely there.
8. Hostility. Mars is a hell of a hostile place for complex Earth-based organisms such as ourselves to survive. There might be some micro-organisms living on Mars, or some that may have lived there in the past, but we didn't evolve to survive in the conditions existing on the surface of that planet. The amount of life-support technology necessary to sustain a human "colony" is frighteningly huge. And frighteningly expensive.
9. Gravity. Mars gravity is about 1/4 that of Earth. It cannot possibly sustain a nitrogen/oxygen atmosphere, or sufficient atmospheric pressure of any gas to allow people to go about without space suits.
10. Oxygen. On Earth, free diatomic oxygen, the stuffe we breathe that sustains our metabolism, exists only because plants produce it via photosynthesis. Oxygen is intensely reactive, and without continual replenishment, would disappear from our atmosphere very quickly. Generating it chemically can be done, but not on the scale necessary to sustain any significant population. You need plants. See several of the items noted above.
In short, we are never going to "terraform" Mars. We might send a scientific team there to explore a bit, but the concept of a continuous colonial occupation of that planet is the stuff of science fiction. I like science fiction as entertainment and food for thought, but I don't conflate it with reality. Even the concept of a manned scientific exploration of Mars runs hard against the wall of expense in a way I see difficult to overcome. Plus we've now had astonishing success with robotic missions, themselves very expensive, but being so successful that they render the necessity of sending actual humans there less pressing.
We are part of the planet we live on. We are here because this planet has the conditions in which we can survive and prosper. Earth wasn't created for us; we came into being because Earth is the way it is, chemically and physically. We are part of it. Earth is the planet we need to be most concerned about. Mars is no refuge for human existence. If we fail to make it on Earth, we, as a species, just plain fail. There are no guarantees that Homo sapiens will succeed in the long run of organic biology on this planet.
caw