There's two basic things you want to work out transits: actual tables for where things are, and a porkchop plot generator to get true time and delta-V requirements. Last I looked at this there were various digital orrery programs that would help with the first, but not much for the second, but current searching did find me
http://sdg.aero.upm.es/index.php/onl.../porkchop-plot.