First step: send an unmanned spacecraft on a round trip to Mars
2018 or 2020
Before we send people to Mars, we have to prove that we can send a spacecraft to Mars -- and back. Every 26 months, Earth and Mars sidle up to each other in their orbits, so logistically it’s best to launch missions when our planets are closest. The next time this will happen will be in 2016, which is too soon in SpaceX’s development to expect them to be ready. But come summer 2018 when we’re in range again, the time will most likely be right for Space X to make an attempt.
Once we get there and back, cargo dumping from other launches can begin to prep the planet for human habitation. Think water, materials for shelter, the tools to convert Martian compounds to oxygen, and fertilizer to grow crops. This all sets the stage for the next step.