There is no known life on Mars, though some are convinced by tests by Mars probes that NASA and others deemed inconclusive. It seems clear that Mars was previously habitable, but clear proof that it was actually inhabited has yet to be found.

There are many possibilities for where to look, which NASA and other space agencies will be investigating in coming years. Perhaps one of the most promising is underground and near methane plumes (on Earth, some forms of life produce methane).

There is some oxygen in the Mars atmosphere, but much less than on Earth. The fraction of the Mars atmosphere that is O₂ is only ~0.1-0.2%. This is much lower than the fraction of oxygen in Earth's atmosphere (~20%).

In addition, the average atmospheric pressure, at ~610 Pa, is also much lower than Earth's 101,000 Pa.

No one knows how long it will take to put humans on Mars. Mars One (a non-government group in the Netherlands) is planning to do so by 2023. SpaceX was founded with the goal of putting humans on Mars, and still has that intent.

NASA is under a presidential order to land humans on Mars by 2033; other government agencies have indeterminate plans to send humans to Mars eventually but with no clear timelines.

Oxygen is a necessary condition for human to survive on Mars, but it is not a sufficient condition. At least fresh water is another important factor. For an entire human society/community to live on Mars, I think we need a build an entire healthy and sustainable ecosystem. I personally do not think that can be done in the foreseeable future. But visiting Mars for a short period of time as a tourist should be much easier, and I think that is achievable in the next few decades, although a round-trip flight will probably cost a lot.

Anywhere off Earth, we'd need to bring our own environmental systems to live in. Mars has hardly any atmosphere, and there is no oxygen for us to breathe.

Maybe there was simple life on Mars long ago, but there is no life on Mars now. That's the opinion of many scientists who study Mars.

The oxygen content of the Martian atmosphere is 0.174%, and that is after taking into account the fact that the Martian atmosphere as a whole is less than 1% as dense as Earth's atmosphere. This oxygen is thought to be created by the sun's ultraviolet light striking ice and stripping the hydrogen atoms off of the oxygen; we do not know of any life on Mars, and Mars would not be livable to humans or any other animal. Mars may have life on it, but the probes that we have sent there have not yet found any.

How long it will take for humans to get to Mars (and if it ever happens) is more a question of politics rather than science. The voyage would take about eight months, but it would be very expensive and quite dangerous, so convincing a government to pay for it, and also risk astronauts' lives, is not easy.

As more experiments are performed and better spacecraft are designed, the trip to Mars will become less dangerous and less expensive, but as far as we know it will always be expensive and never truly safe.

We do not know if humans will ever be able to live on Mars, even if we could build cities with air-tight domes that would keep a breathable atmosphere in. The reason is because Martian gravity is about 1/3 that of Earth. We know that humans cannot live in space forever in zero gravity: people on the ISS (International Space Station) need to be brought back to Earth in a year or so or they will eventually die from the loss of muscle that happens in zero gravity. We don't know how much gravity humans need, and if Mars' 1/3 of Earth's gravity is enough.

It will be possible to build a space station in orbit around Mars that humans could live on, however, provided thatthe space station were rotating to produce artificial gravity (the ISS does not rotate and so lacks artificial gravity).