Je suis le roi.
Here's a breakdown:
* Je means "I"
* suis is the present tense of the verb "être" (to be)
* le means "the"
* roi means "king"
You can also say:
* C'est moi le roi. This is more emphatic and means "It's me, the king!"
* Je règne. This translates to "I reign" and implies you are the king.
The best way to say it depends on the context and tone you want to convey.