Colonizing Mars is quite a challenge. Maybe a first step would be to colonize the oceans and then take that technology to other planets. It might be easier this way because a lot of the challenges you face on another planet (hostile environment, atmosphere, isolation) you can find in the deep here on Earth. And when you had enough you can always come back to the surface whereas on another planet transportation might be a bit more tricky.

The biggest challenges on Mars are probably pressure (the atmosphere on Mars is very thin), magnetic field (protects from cosmic radiation), atmosphere (terraforming perhaps).

Another big issue might be indigenous life. Throughout Earth's history there have been multiple (132 to be more precise) small meteors that landed here on Earth that came from Mars. It is concisely possible that the asteroid that killed the dinosaurs (or another one perhaps) projected a piece of rock from Earth to Mars containing Earth microbes or bacteria. These can be either dormant or active and they can pose a big hazard to humans (having their evolutionary paths so different from our).