How to get to other planets? Stock parts only. Planet suggestion?

Duna is the easiest, there is a very weak atmosphere so you can use parachutes but just them is not enough.

For getting there, theoretically you can start at any time, but it's best for fuel usage and time needed, to start at some specific times, that are called transfer windows. You can calculate them using mods, like mechjeb.

For the rocket, the hardest part is as always take it into earth orbit, this process requires a lot of fuel because a rocket capable to go to Duna is usually quite heavy at least.

After getting into earth orbit probably the best option is to use a nuclear engine, that is very efficient, although not very potent.

After having escaped earth gravity field you have to find the right maneuver node to intercept the planet, execute it, retrograde burn when on the closest point to the planet until you're in orbit and you're done.

You can also aero brake on planets with atmosphere by just placing your periapsis inside the atmosphere so that the drag will do the work that the engine should do and use less fuel. Remember to bring the periapsis up when you have the right orbit

/r/KerbalSpaceProgram Thread