My mobile phone is Nokia 5300. How can I lock the SMS?

You can use third-party software, such as program encryption software, which generally requires signing or cracking the phone.

The easiest way: press the power off button and a dialog box will appear. Select the penultimate "Lock Phone" and enter the password. The original password was 12345. If you need to change the password, you can change it here in Settings-Mobile Phone-Mobile Phone Management-Security Settings-Mobile Phone and SIM Card-Lock Code!

Change the automatic lock time: You can set the required time in Settings-Mobile Phone-Mobile Phone Management-Automatic Keyboard Lock.

I hope I can help you!