Most people just work their way down and along the southern coast from Istanbul, common stops are Fethiye, Kas, and Olympos but there are many more towns with nice beaches. Wherever you stay, they’ll recommend their "brother’s" or "cousin’s" guesthouse wherever you’re headed, so you can go with that, or try places in the guidebook or just turn up and go with someone at the station. Try and make time for Cappadocia in the interior if you can, it’s great.