Using Scenario editor -
1. Make sure "ScnEditor" is Checked under the "Modules" tab of the Launcher.
2. Launch any scenario. Then Press F4 -> "function" -> Scenario Editor "OK" -> Edit -> Orbiter Elements.
3. Change Orbit reference to say "Mars".
4. Change Semi-major axis units to "Planet rad." and use a value like 1.5. Set other values in the left column to zero.
You are now orbiting Mars!
Here's a video to understand the 6 orbit parameters you can set in the Scenario Editor:
To actually fly to Mars, there are some excellent tutorials on youtube. Check this out