Finish rocket. NASA's extremely powerful Space Launch System won't fly until late 2018 or early 2019. That wouldn't provide a lot of time for test flights before we'd use it to hurtle astronauts on their very long and hypothetical journey to the red planet in 2020.
Make sure no one's gonna die. The ride to Mars is probably going take six- to nine-months. That's a lot of time for a human body to suffer though weightlessness, which destroys our bones, and deep space radiation, which causes cancer and messes with our brains and possibly our hearts. Not to mention the risks of going bonkers from boredom.
Build space RV. Speaking of going bonkers, can you imagine spending several months crammed into this tiny spacecraft with your fellow astronauts? No, because the Orion capsule is meant for short trips. For longer trips—like, Journey to Mars long—astronauts are going to need a bigger habitat with radiation protection, life support systems, water recycling, food storage, and a working bathroom. NASA hasn't yet selected a design for the habitat, let alone started building one.