Travelling by car
From Central Europe getting to Turkey is not too difficult. In any case you'll need your International Insurance Card (Green Card). Pay attention to "TR" not being cancelled and be sure your insurance is valid for the Asian part of Turkey, too. Otherwise you will have to buy Turkish car insurance.
Major roads from Europe are:
E80 enters Turkey at Kapıkule border gate (NW of Edirne, SE of Svilengrad) from Bulgaria
E87 enters Turkey at Dereköy border gate (north of Kırklareli, south of Tirnovo) from Bulgaria
E90 enters Turkey at İpsala border gate (east of Alexandroupolis) from Greece
Major roads from Middle East enter Turkey at numerous border gates around Antakya (Antioch), from Syrian cities such as Aleppo and Latakhia, Habur border gate (south of Silopi, north of Zakho) from Iraq, and Dogubeyazit border gate (near Ararat) from Iran.
Major roads from Caucasia enter Turkey at Sarp border gate from Georgia (south of Batumi) and Türkgözü border gate south of Akhaitsikhit (this is the nearest border gate from Tbilisi). The border with Armenia is currently closed, thus impassable by car.
There are also other border gates (unlisted here), from all the countries Turkey has a common land border with (except Armenia), leading to secondary roads passable with a car.