Tips (gratuities) are generally modest in Turkey (a few percent of the price paid), although Americans tend to do their thing and tip big, which is leading Turkish tourism industry workers to expect big tips from Americans who travel to Turkey.
Although the person you tip would prefer Turkish liras, you may tip in any currency so long as you give notes/bills (paper money). Don't give non-Turkish coins, as these cannot easily be exchanged for Turkish liras.
RESTAURANTS: Small tips (5% to 10%) are not necessary, but are appreciated in inexpensive establishments. In luxury restaurants, tip 10% to 15%.
HOTELS: Porters are happy with 50¢ to US $1 per bag.
ARCHEOLOGICAL SITES: If a site guardian performs some special service, such as giving you a personal tour, a tip equivalent to a few dollars is appreciated.
TAXIS: For taxi drivers, don't tip, just round the fare upwards to a convenient amount. So if the fare is YTL4.8, round it up to YTL5.
DOLMUS/MINIBUS: No tip expected.
AIRPORTS/STATIONS: Every airport, bus station (otogar) and train station (gar) has an official tariff for porters, which should be posted prominently. Of course you probably won't see it, so tip about 75¢ per bag, which should be plenty. If you actually end up underpaying according to the official tariff, the porter is sure to let you know!
TURKISH BATH (HAMAM): At the end of your bath all the attendants (and there are lots of them) will line up to "bid you goodbye" and expect a tip (if service has been good). Share out among them about 15% of the total price of the services you've used.