Generally speaking, entering Turkey by air is very straightforward. The only complication to be aware of is that most people need a ‘visa’ which is really just a stamp in their passport issued at the point of entry. If you fly into the country you must first join the queue to pay for the stamp in your passport before joining the queue for immigration. Rarely do customs officers stop you to check your bags.
Entering the country by land can be a little more difficult. Getting a visa is the same procedure as by air, but sometimes you can only pay for the visa in Euros or US dollars. And at many of the land border crossings there are no facilities for either changing money or ATMs to access cash, so make sure you bring enough money to pay for your visa. You may also want to consider having some Turkish Lira (TRL) on you before you get to the border.