Almost all Turkish citizens use the e-devlet application, where all public services can be used online, from paying taxes and fines to making doctor's appointments. Foreigners who have obtained a residence permit can also register on the app. There are two ways to do this: through the PTT post office and through online banking (if you have a Turkish bank card).
Registering through PTT branch
You need to have with you: an ikamet card and a cell phone with a Turkish SIM card. You need to show the PTT employee your ikamet and ask him to give you an "e-devlet şifresi". After you pay 4 lira, your phone will receive a text message with a code, which you need to dictate to the postal employee. Then you will receive another SMS with a code, which you do not need to show to the employee. The second code-password you enter when registering at https://www.turkiye.gov.tr/ by clicking on the "Giriş yap" button in the upper right corner of the page. After successful login, it is advisable to change your password to a new one.
Registration via Internet banking
Go to https://www.turkiye.gov.tr/, open the tab "Internet Bankacılığı" and there you need to enter your İkamet or card number and your Internet banking password. Some banks allow you to log in via QR code. After successful activation you will be redirected to the e-devlet page with your account, where it is better to change your password.