Formal:
* Ma vie est ennuyeuse. (This is the most literal translation and is appropriate for formal situations.)
Informal:
* Ma vie est chiante. (This is a more colloquial way to say it, meaning "My life is a drag.")
* Je m'ennuie dans ma vie. (This means "I'm bored with my life" and is a more emotional way to express it.)
* J'ai une vie monotone. (This means "I have a monotonous life" and emphasizes the repetitive nature of your life.)
* Je suis blasé(e) de ma vie. (This means "I'm bored with my life" and expresses a feeling of disillusionment.)
More colorful expressions:
* Ma vie est un vrai calvaire. (This means "My life is a real ordeal" and emphasizes the negative aspects.)
* Ma vie est comme une soupe. (This means "My life is like soup" and emphasizes its lack of excitement.)
The best way to say it will depend on the context and your relationship with the person you are speaking to.