Hey ! Scott manley has a very good tutorial about how to get to planets outside Kerbin's SOI : https://www.youtube.com/results?search_query=scott+manley+how+to+get+to+duna
AND there's this very useful website that tells you where you should be relative to the planet you want to go to , just select the planet and it will show you whether you need to be about 90 degrees from the sun or more or less. here you go
http://ksp.olex.biz/
Good luck !