Formal:
* Je souris. (This is the most common and literal translation.)
Informal:
* Je fais un sourire. (This literally translates to "I'm making a smile.")
* Je suis tout sourire. (This translates to "I'm all smiles" and emphasizes happiness.)
You can also use different verbs to describe the smile:
* Je ris. (I'm laughing - indicates a bigger smile)
* J'ai le sourire. (I have a smile - implies a gentle, soft smile)
The best choice depends on the context and the tone you want to convey.