I'd never seen or heard of that website until today. I don't know what the rest of it's like, but if it's suggesting you use "Listen me up!" then I'd be inclined to stop using it.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
To sum up, these two are oaky:
Listen to me!
Listen up, everyone!
NOT: Listen me up
NOT: Listen up me
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.