Hello @Lenka1, welcome to the community! Your query was translated to English, since this is a international forum.
What Android version do you have in your phone right now? You can see your phones Android version if you go to Settings > About Phone > Android version.
In Android 6, you can choose a MP3-file as ringtone if you go to Settings > Sound and Notification > Phone Ringtone, and then look for the + in the top right corner. This will take you to the MP3s stored in your phone and you can choose your favourite song.
Let me know how it goes, and have a nice weekend!
