Formal:
* Ek is baie gelukkig. (This is the most literal and formal translation)
Informal:
* Ek is baie bly. (This is a more common and casual way to say it)
* Ek is super gelukkig. (This emphasizes the happiness)
* Ek is so gelukkig. (Similar to the previous, but a bit less intense)
You can also use a more emphatic version with the adverb "regtig" (really):
* Ek is regtig baie gelukkig.
* Ek is regtig baie bly.
The best option for you will depend on the context and the person you are talking to.