Pretty much the same as go-faster has advised, take cash and change it over there.
Avoid banks and change offices, instead, find a corner shop and you will find they give you a better rate and no commission or charges. Change money at the start of the week if possible as rates tend to go down toward the weekend when the banks are shut.
Turkish people are mostly friendly and helpful, avoid street sellers and you won't go far wrong, local councils are clamping down on restaurant staff hassling you to come in, but if they do, be polite and say "no thank you", dont make the mistake of saying you will come in tomorrow, they WILL remember you and claim to be offended next time you pass.
Enjoy your holiday, it's a great place.