Formal:
* Je suis heureux/heureuse. (This is the most literal and formal translation.)
Informal:
* Je suis content/contente. (This is a more casual way to say "happy.")
* Je suis joyeux/joyeuse. (This emphasizes a feeling of joy and exuberance.)
* Je suis ravi/ravie. (This is a very enthusiastic way to say "happy.")
* Je suis enchanté/enchantée. (This expresses delight or pleasure.)
You can also use verbs that express happiness, like "être heureux" (to be happy) or "se réjouir" (to rejoice).
Example:
* Je suis très heureux de vous rencontrer. (I am very happy to meet you.)
* Je suis contente d'avoir passé une bonne journée. (I'm happy to have had a good day.)
The best way to choose the right expression is to consider the context and the level of formality.