This is a very common expression that you're bound to hear. It's the short form of No te metas tu cuchara en esto - Literally it means "Don't stick your spoon in this". Or as we would say in English, "Mind your own business". But that's not the only way you can use this expression. Let's look at some examples:
No te metas en mi vida - Stay out of my life
No te metas con mi amigo - Don't mess with my friend.
No te metas con Zohan - Don't mess with the Zohan
No te metas conmigo - Don't mess with me
No te metas en mis asuntos - Stay out of my business, mind your own business
No te metas donde no te llaman - Don't intefere when nobody asked you
Ask you can see, the meaning changes slightly based on the context, but I think (hope) the examples give you a good understanding of both how to interpret and use this very common phrase.
It's also very common to see and hear the short verstion. If someone is getting a little to interested in your asuntos (affairs, or business), you can tell them "no es asunto tuyo" or simply, "no te metas". And if you happen to be the recipient of this message, well, you've probably overstepped your boundries.
And like most things in language, your tone of voice can change this from a very strong statement to something fun and playful in the right situation.