How do I get notified when somebody tweets on Twitter?

[Sarah]

I have an iPod Touch 4G, and I want to be notified when One Direction/other tweets. There's a way to do it if you have an iPhone, like it'll text you when their tweet. And I know there's a way you can do it with an iPod Touch too, where instead of it sending a text to you, it will pop up on your screen, like when somebody tweets you. Please help me thanks.