How do I keep variables when game is closed?

I'm quite new to Unity and I'm making my first game. In this game player gets through levels and unlocks new abilities, then it is possible to replay levels with new skills. I need to keep few variables (last played level, characters unlocked skills, unlocked levels) from beeing deleted when player quits the game and gets back. I've already searched through the internet but I still have no idea how to keep data from beeing deleted. I guess I need some kind of save file but how do I make one? Please help. Also, I write in C#.