Formal:
* No se preocupe. (This is the most common and formal way to say it.)
Informal:
* No te preocupes. (This is the most common and informal way to say it.)
* No te preocupes por eso. (This is a bit more emphatic, and means "don't worry about that.")
* No te preocupes, todo va a estar bien. (This means "don't worry, everything will be alright.")
Slang:
* Tranquilo. (This literally means "calm," but it's used as a slang term for "don't worry.")
* No pasa nada. (This literally means "nothing happens," and it's a common way to say "don't worry" in a casual setting.)
The best option for you will depend on the context and your relationship with the person you're talking to.