If you're flying into Heraklion, then you can get a cab from the airport to the harbour bus station (10min), then a bus to Rethymnon (one and a bit hours) and from there to Plakias (45min) - last time I did it, it was about 25 euros in total. However, there aren't a lot of Rethymnon to Plakias buses, especially not as early as May 1. Google KTEL Crete for timetables. It's a good introduction - locals on the buses and the fabulous views of the gorge just before you get to Plakias.