Practical Info:
The currency in Turkey is Turkish Lira (TL). One Euro is approximately 2 TL. Most places accept credit cards. You can withdraw money from ATMs. The rates at the airport is not that bad (after you get out of the luggage area). there are many small shops that says ‘Döviz’ which could also exchange Euros.
If you travel by taxi make sure that they turn their taximeter on. There is no price difference for day and night. You can round to the next reasonable number (27 to 30 TL for example), around 10% tip is ok. Same for restaurants, around 10% tip would be good. If you need a receipt, ask for a "fiş" pronounced exactly like the English word "fish".
Weather is very nice, but there is some chance of rain for 21nd. 22-24 seems clear, with very little chance of rain.
http://www.weather.com/weather/today/TUXX0014
You can get tokens for the tram. If you are staying for the weekend and travel more I can help you get an Akbil which you can use for tram, all busses and ferries. You pay for the akbil itself and then fill it so it only worth if you are staying more.
That’s all I can think of right now, see you later.
Travel:
Istanbul has two airports. The larger one is the Ataturk Airport which is at the European part of Istanbul. It is much easier to travel to Karakoy and Sultanahmet area from this airport. Taxi should cost around 50 TL or less (25 Euros). Or you can take the metro to Aksaray, and from Aksaray you can take the tram to Sultanahmet and Karakoy. Alternatively you can take HAVAS buses to Taksim and then take a taxi to your hotel.
Most hotels have airport pickup services, however, it may require minimum number of stay. Ask your hotel.
The smaller one is the Sabiha Gokcen airport at the Asian part of Istanbul. Taksi from that airport will cost more than 100 TL (around 50 Euros) and you will hit horrible traffic on the bridges depending on your arrival time. Unless you have compelling reasons to do so, I do not recommend using this airport.
Visa:
Citizens of some countries are required to hold a valid visa to enter Turkey. Note that for majority of the cases visas are obtained in airports in Turkey, and no paperwork is needed. Just carry enough dollars or euros in your pocket. This table shows the countries for which a visa can be obtained at the airports in Turkey, and the respective visa fees.
For detailed information, click here.
If you need invitation letter for visa, let me know asap. The visa fees can be paid in Euros.