The official currency in Turkey is the New Turkish Lira (YTL – Yeni Turk Lirasi) which is sub divided into New Kurus (Yeni Kurus) 100 YKr = 1 YTL, Images of the Notes and coins can be found here. http://www.tcmb.gov.tr/ytlkampanya/banknotes/banknotes.htm
The YTL is internationally traded and can be purchased in most countries prior to your visit; although you may need to order this ahead.
Once in Turkey, it's very easy to change all major currencies in Banks, exchange offices, post offices (PTTs) and many Hotels.
Currency can be also obtained from the major debit or cash cards from many ATM machines. (Check the logos on the ATM machines)
However if you do run short of Turkish Lira at any point during your stay all the major foreign currencies, are widely accepted in many shops, hotels and restaurants especially in the tourist areas – be sure you know the exchange rate though.