Je suis amoureux/amoureuse.
* Je suis amoureux is used by men.
* Je suis amoureuse is used by women.
Here are some other ways to say "I'm in love" in French:
* J'aime beaucoup [person's name]. (I love [person's name] very much.)
* Je suis fou/folle de [person's name]. (I'm crazy about [person's name].) *Fou* is used by men and *folle* is used by women.
* Je suis éperdument amoureux/amoureuse de [person's name]. (I'm madly in love with [person's name].)
* Je suis tombé/tombée amoureux/amoureuse de [person's name]. (I fell in love with [person's name].)
* [Person's name] est l'amour de ma vie. ([Person's name] is the love of my life.)
Remember: The most appropriate way to say "I'm in love" will depend on the context and the relationship you have with the person you're talking to.