The dressing should be a simple lemony-vinaigrette (sans vinegar) type to let all the flavors shine through. Here is one I'm sure you'll love...and while not technically authentic Greek (due to the seasoning added), it's nearly perfect.
Perfectly Delicious Greek Salad Dressing
4 TBSP fresh lemon juice (if you like it super lemony, add another)
1/2 cup olive oil (EVOO)
1 TSP. Cavender's Greek Seasoning (I keep this stuff on hand for many, many uses http://www.greekseasoning.com/)
1/4 TSP. dried mint AND 1/4 TSP. dried oregano
Whisk or blend together and voila!