NASA has been working on plans to send humans to Mars for many years. Over the last several decades they have sent orbiters, landers, and rovers to Mars in preparation for human exploration.
In order to get people to Mars a number of challenges must be overcome and there are stages in the development.
1. Earth Reliant
The first step to getting people on Mars will be exploration and research on the International Space Station. This will allow astronauts to learn what it is like to live and work in space for long periods of time.
2. Proving Ground
Next a series of missions near the moon will test what is needed to live and work on Mars. The proving ground will be days away from Earth paving the way for the amount of time astronauts will be from Earth when they are on Mars. A yearlong mission is in the works that will send astronauts farther than any humans have ever traveled before.
3. Earth Independent
At this point, tests will be completed to learn about entry, descent, and landing needed to land safely allow humans to walk on Mars.
At this point in the process there are some Earth Independent missions taking place, but they are robotic missions with the rover due in 2020.
The projected timeline for humans to travel and orbit Mars in in the early 2030s.