Unlocking levels is a simple global variable, can be a bool, true/false etc. When tutorial is complete, set level 1 locked to false. When level 1 is complete, set level 2 locked to false etc. To save this information you can use system Save feature which will Save the state of the game at that point, essentially saving what is unlocked. If you're saving scores and things you can look into the local storage style save system.