I can highly recommend having a play with Kerbal Space Program to get an intuitive grasp of orbital mechanics.
Basically, if you don't go fast enough, you're going to come back down to earth. Once you get to a speed that's fast enough that you aren't going to come back to earth, you'll find you are now in orbit around the sun, in a path remarkably similar to the one the earth is on. To get to Mars you need to get up into a higher orbit around the sun, so you need to go even faster.
Basically, if you don't go fast enough, you're going to come back down to earth. Once you get to a speed that's fast enough that you aren't going to come back to earth, you'll find you are now in orbit around the sun, in a path remarkably similar to the one the earth is on. To get to Mars you need to get up into a higher orbit around the sun, so you need to go even faster.