The ISS is mainly a space laboratory. The astronauts will study medicine, Someday, astronauts and their families may move to Mars. To make Mars able to have people live on it, astronauts would use a process called terraforming. This process would slowly fill Mars' atmosphere with oxygen, make precipitation and bodies of water, and have plants grow on Mars. Here are the terraforming steps.
Astronauts go to Mars and set up biospheres.
These are domes with a fake atmosphere of oxygen. They would grow food in the biospheres. The actual process of terraforming starts by making machines to pump greenhouse gasses into Mars' atmosphere to trap heat. Clouds will start to form and the Martian atmosphere will slowly begin to turn from red-orange to blue with oxygen. After a while, when there is more oxygen, more astronauts will come and build cities.