There are daily ferries from Athens/Piraeus to Santorini. The two most popular ferry companies are Blue Star (regular ferry) and Hellenic Seaways (highspeed ferry). Both have websites. Blue Star takes 8 hours, HS takes 4.5 hours. Blue Star costs about 35 euro for an economy ticket, HS costs around double that. Schedules for next June won't be available until late April.
Aegean Airlines flies to Santorini from Athens in about 45 minutes. If you keep an eye out on their website you may be able to catch a discount promo fare of 39 euro, all taxes included.
If you want easy walking-distance access to the island's youth-oriented nightlife focus your hotel search on Fira and Firostefani. If you want peace and quiet look in Imerovigli. If you want the most beautiful and timeless village focus on Oia. If you want to stay at the beach look in Perissa Beach or Kamari Beach.