If you're entering or leaving Istanbul by car, E80 runs between Istanbul and central Anatolia to the east; this toll road is the best of several alternatives. You can also enter or leave the city on one of the numerous car ferries that ply the Sea of Marmara from the Yenikapi docks. Getting out of the city by car can challenging, as the signs aren't always as clear as you would hope. It's always useful to have a driving map.
Istanbul is notorious for congested traffic, cavalier attitude to traffic regulations, poor signposting, and shortage of parking spaces. In short, don't even think about renting a car for travel in the city.