Formal:
* Tudo bem, meu amor. (This is the most literal translation, but it sounds a bit stiff.)
* Não se preocupe, meu amor. (This means "Don't worry, my love.")
* Está tudo bem, meu bem. (This means "Everything is okay, my dear.")
Informal:
* Está tudo bem, amor. (This is the most common way to say it.)
* Deixa pra lá, amor. (This means "Let it go, love.")
* Relaxa, amor. (This means "Relax, love.")
More affectionate:
* Calma, meu bem. (This means "Calm down, my dear.")
* Fica tranquilo, meu amor. (This means "Be at ease, my love.")
The best way to say it depends on your relationship with the person you're talking to. If you're speaking to someone you're close to, you can use the more informal options. If you're speaking to someone you don't know very well, it's best to use the more formal options.