Employment opportunities for foreigners are limited in Turkey. Foreigners are forbidden to hold jobs in the legal or medical professions, and much of the work that is available pays poorly.
The same law that prevents foreigners from working in law or medicine also regulates which products they are allowed to sell if in business for themselves. This is because certain products (such as fixed phone lines) are considered Turkish government monopolies, though presently many of these monopoly products are being de-regulated.
Foreigners who are married to Turkish citizens or who have obtained Turkish citizenship are not restricted by the employment laws, though their options will still be limited if they do not speak fluent Turkish.
Most foreigners working in Turkey arrive with employment secured. Many foreigners are assigned to Turkish branches of foreign companies, foreign embassies, or NGOs. Those that are not might have a tough time earning a living. Sectors of the Turkish economy with the most openings for foreigners are teaching and tourism.
Language teaching jobs are plentiful throughout Turkey, and while they do not pay extraordinarily well, they offer enough money to survive. If you are a fluent English speaker, you may also be able to work giving private English lessons (you will earn around 30 Turkish lira an hour).
Working in tourism is tougher financially: while English-speakers are in demand, most employers (at least in restaurants, bars, and other shops) will not offer much more money than it takes to cover the cost of living. Jobs in hotels and at resorts pay better but are less common.
Some foreigners who know the country and have the capital opt to open tour agencies of their own. While many of these are successful, like all small business ventures, they are not for the easily-discouraged or faint-hearted.
Foreigners can also be found working as journalists and nannies in Turkey, though journalism jobs are relatively uncommon and nanny positions can be extremely demanding.
Regardless of where you want to work, fluency in Turkish will be a major asset, if not a job requirement.