What are some of the problems that we most overcome before our expedition to Mars in 2018?

Answer 1

Finding the money first of all.
3 major practical difficulties:
Current engine technology isn't efficient enough to carry a large payload over such a large distance in a sensible period of time. Something like a nuclear rocket would do the job, but that technology is still at an early experimental stage.

The crew need food, water and air for the endurance of their trip. This means either carrying vast stocks of it, or learning to more efficiently recycle it. A suitably efficient system to recycle food, water and air has yet to be successfully developed.

The solar wind. If a solar flare occurred at any point during the voyage, as it almost certainly would, then in deep space the resultant radiation could prove fatal to the crew unless effective radiation shields had been fitted to the ship. This can be done, but it adds significantly to the weight.

Answer 2

That's just over ten years away. It will never happen. We'll be lucky to get people back on the moon by then.