A great question, and there’s no easy solution to this critical problem!
The sun gives off deadly radiation all the time. On Earth we are shielded from it here because of our magnetic poles and this is what causes the Aurora borealis (northern lights).
On Mars this will be a deadly problem whether or not there would be an atmosphere like Earth.
The best solution, like on the International Space Station, stay indoors as much as possible!
Besides, I don’t think we will be able to terraform Mars!