You will need some Turkish money for small purchases etc but be careful, the money is VERY confusing as a result of massive inflation. One British pound is worth more than 2.7 million lira! Write the value of the various banknotes on a piece of paper and do not confuse 500,000 lira notes with 5,000,000 ones etc.
Guide books will probably tell you that US dollars are widely accepted, but these days euros are better and most things can be paid for in euros.
I don't know what part of Turkey you are going to, but in Istanbul, hard negotiation can produce great results in restaurants. Tourist numbers are down and they need our money. For example, at a restaurant at the fish market, my wife and I were shown a menu and asked what we wanted to eat. We chose a starter, main fish course, two beers, and a dessert. The "tout" offered �20 for the two of us including cover charge. When the bill came, it said simply "2 meals special price �20".
Haggle, haggle, haggle.
Because euros are so widely accepted, you might be asked to change coins into bank notes. It would be kind to do this to help them out so that they can change the euros at the bank.