It means "I speak bad English but I think I'm cool". What they should say is: "How are you?".

It means "I have bad grammar"

What's doing? < very bad grammar - not street slang I've never heard of this (in the USA). The person should say "What's up?" or "What's going on"? If that is what they mean to ask.

Thanks to both! What's cooking/cracking/poping... Sounds interesting~ =D

Means 'what's going on', 'what's cooking'. Like 'how's going' vs 'how's it/everything going'.