One day we will be able to, but there are a lot of issues before getting there.
As Steven has mentioned, terraforming is one proposal for making mars better for us to live upon.
Another option is to create biospheres. Biospheres are sealed environments that carry out the same functions of what our environment does here on earth. Think of a football stadium, with a sealed roof, and we would live in there with plants and others things growing to keep us going. Connect a few of these together and it is possible, but it is very costly and there is still a lot of work to go to make it work properly.
I disagree with Steven about terraforming being environmental vandalism but then I’m a bacteriologist and I like the idea of bacteria opening up another planet for human expansion to in the future. You know when we finally break this one.
While difficult it is certainly possible as Mars has some very similar properties to Earth and has apparently similar soil, available water and an abundance of carbon dixoide in the atmosphere and frozen on and below its surface.
While I don’t think we will be going there any time soon, it seems it is possible and may be something we are forced to look at quite a while down the track.