NASA once said that no private company's reaching the red planet without its help. If that's true, then private space corps should be thankful that the agency has a solid plan to get us there. America's space agency has published a document that details the steps it's taking to reach Mars. In it, NASA outlines the three phases of its journey, starting with a step called "Earth Reliant," which is comprised of conducting experiments aboard the ISS and studying how long-duration missions affect the human body. This phase is already ongoing, with the agency testing out different materials and 3D printing on the space station and conducting appropriate research on human behavior and health for the first batch of lucky astronauts.
The next phase is called "Proving Ground." As the name implies, it's the stage when NASA needs to put its technologies to actual use and prove that they work. SLS' and Orion's first flight together (scheduled for 2018) is part of it, along with the 2020 Asteroid Redirect Robotic Mission. The latter aims to take an asteroid chunk to the moon's orbit in order for the agency to test the new technologies, like Solar Electric Propulsion (SEP), it plans to use for future Martian missions.
For the third and final stage called "Earth Independent," NASA wants to apply the technologies it's developed and tested to send astronauts near the vicinity of Mars and its moons before (finally) landing on the planet's surface. The agency expects to have developed the technology needed for humans to live on the Martian surface by then and to harvest resources from the planet, such as oxygen, water and fuel. You can see an overview of the three-stage plan below, but if you need something long to read this weekend, you can find the full document (PDF) on NASA's website.