My best friend and I (both age 17) will be traveling to Athens, Paros, and Santorini June 21 to July 6th. We have two questions: 1. Should we purchase our ferry tickets from Athens to Paros, then Santorini and back to Athens before we leave the states or wait until we get to Athens? Will the ferries fill up at that time of year? 2. Should we book our rooms when we get to the islands or on the internet now?
You can wait on the ferry tickets. The car ferries are huge and won't be full. You can save time at the ferry port of Piraeus by buying your tickets at the airport from the travel desk of Amphitrion Holidays in the Arrivals Hall between doors 2&3.
If you want to wait on the rooms you can as long as you're willing to take what you can get when you get there. There will be cheap rooms offered by people meeting the ferries but many are not in the best locations. If you have a specific area of an island in mind it might be best if you do some research and book ahead so you get what you want.
Also, if you don't already know it, the island of Ios, which is between Paros and Santorini, is the #1 Greek island party scene for backpackers.