English is widely spoken in İstanbul and touristy parts of western Turkey; less so in eastern and central Anatolia, where knowing a few Turkish phrases, covering relevant topics such as accommodation, is invaluable. Turkish is fun to learn as pronunciation is easy. Learning Turkish is more useful than Kurdish, as most Kurds speak Turkish (but not vice versa). Many Turks speak German.
İstanbul is the most popular place to learn Turkish, though there are also courses in Ankara, İzmir, Antalya and other places. Try to sit in on a class before you commit, as the quality of your experience definitely depends on the teacher and your classmates.
Private tuition is more expensive, but tutors advertise at http://istanbul.en.craigslist.com.tr and the expat website www.mymerhaba.com. Many books and online resources are available; the books and CDs by David and Asuman Pollard in the 'Teach Yourself' series are recommended.
Schools include the following:
Dilmer (www.dilmer.com) İstanbul.
International House (www.turkishlesson.com) İstanbul.
Tömer (tomer.ankara.edu.tr) Ankara, İstanbul, İzmir, Antalya, Adana, Bursa and Samsun.
Turkish For Foreigners (www.spokenenglishtr.com) İstanbul.
Turkish Language Center (www.turkishlanguagecenter.com) İzmir.