If you want to stay in Turkey for more than 90 days, you will have to apply for a residence permit. If you have a valid visa, you only have to apply for a residence permit if the visa expires during your stay and if your stay exceeds 90 days.
There are several types of residence permit:
You should apply for a residence permit at the Turkish consulate in your home country or country of residence. If you are already in Turkey, you can exceptionally apply for a residence permit with the local authorities.
If you want to apply for a residence permit, you need a passport/travel document valid for at least 60 days after the expiration of the requested residence permit and two copies of it. Furthermore, you will need at least four passport sized photos (to be on the safe side, bring eight), your visa, if you have one and the filled-in application form . Depending on the type of residence permit, you might need further documentation (e.g. a certification from your employer, school, university, bank, etc.).
Your residence permit will be valid between 6 months and 5 years, depending on the type of permit and the reason for your application. The costs may vary, but round €60/$80.
If you want to renew your residence permit, you have to do so at the local authorities, 60 days before the permit expires. You will be issued a free document stating that your application for renewal is pending and that you are allowed to stay in Turkey until the decision has been made, even if your permit has expired by that date. The new permit will be valid exactly from the expiry date of the former permit.
However, under no circumstances should you attempt to deceive Turkish authorities to prolong the validity of your residence permit - you risk deportation to your home country or a term in Turkey’s infamous prison system.
In certain cases, you may apply for a different residence permit. If one of your parents is Turkish, for example, and you turn 18, you can immediately re-apply for a short-term residency permit. The only requirement is that you must have lived in Turkey for at least three years.
If you get divorced from a Turkish citizen and want to stay in Turkey, you can also apply for a short-term residence permit. You must have lived in Turkey for three years, unless you have been a victim of domestic violence.
However, in both cases, applying for another permit is optional, as the family permit does not expire sooner when you turn 18 or get a divorce.
If you want to apply for a Turkish long-term residence permit, you have to fulfill the following requirements:
You will need the same documentation as for the other residence permits, plus your residency book and your Turkish tax number.