What kind of trading?
In most cases, I'd recommend starting with a local company. Focus on growing the business. If you form a foreign company and don't comply with all applicable tax laws and reporting obligations (both in Turkey and where the company is formed), you're setting yourself up for a lot of headaches in the future.
What advantages do you seek by forming a foreign company? Keep in mind that any foreign/offshore company that you own and control from Turkey is tax resident in Turkey and must pay Turkish tax.