Hell yeah! - How to express great joy?
It depends on what dialect you're writing in. For example, Cantonese (primarily used in Hong Kong) has slang versions that is different from Mandarin.

In Mandarin

I'm a good boy
乖仔 (this is in cantonese and is a slang). It means, "Good boy!"
我很聽話 (this is universal). It literally means, "I listen." But it in practice means, "I'm well behaved."

Hell yeah: 
棒死了 (this means "fresh to death!")
The important word is "棒", which means "awesome."
Alternatively, some people use "酷" as a slang. This literally means "Cool" and sounds like "cool" in Chinese.

I so great
我好勁 
The important word is "勁," which means "strength" or "great"
This is slang term. But if someone read this, they would laugh.