* Il pleut: This is the most basic and common way to say it. It literally translates to "it rains."
* Il fait un temps de pluie: This means "it's rainy weather."
* Il y a de la pluie: This means "there is rain."
You could also use more descriptive phrases like:
* Il pleut à verse: It's raining heavily.
* Il pleut des cordes: It's raining cats and dogs.
* Il pleut doucement: It's raining softly.
The best way to choose the right phrase depends on the situation and the level of formality you want to use.