Cards work almost everywhere. Restaurants, shops, taxis, tram tickets — all take cards, and contactless is normal. Make sure your card is enabled for use abroad; some banks keep that switched off by default.
You still need cash. Small traders, market stalls, some taxi drivers, the simit sellers and tea stands work in cash. 500–1,000 lira a day covers most people.
Do not change money at the airport. The rate there is noticeably worse. Exchange offices in the city give much more and charge no commission. There are plenty in Beyoğlu and Karaköy; comparing the buy/sell rates in the window is enough.
ATMs are everywhere. When you withdraw with a foreign card and the machine offers to "convert at our rate", say no — your own bank's rate is almost always better. The same question comes up when paying by card; the answer is still no, take the charge in Turkish lira.
Tipping. Not compulsory, but usual. Ten per cent in a restaurant is normal, fifteen for good service. If service is already on the bill you need not add more. In a taxi, rounding up is enough. For cleaning, 50–100 lira a day. In a café, leaving the coins is fine.
Taxis. Insist the meter is running and refuse a fixed price. Ordering through an app (BiTaksi, Uber) removes the negotiation altogether.
Where haggling applies. Only in the bazaars and the antique shops. Not in restaurants, shops or taxis — if it is offered there, something is wrong.