Voice mail greetings often employ a lot of idiomatic language, so even if you're good at French it might be tricky to record an authentic-sounding message.
Most French voice mail greetings are recorded in the formal (or "vous") register. But if this is for your mobile phone and you want a more informal message for your friends, consider using the "tu" or informal register.
Here's a typical formal voice mail greeting. "Bonjour! La personne que vous avez appelé n'est pas disponible. Merci de laisser un message après le bip sonore."
Here's a less formal message you can use, which is also very typical. "Salut! Je ne peux pas te répondre pour l'instant. Laisses-moi un message après le bip sonore. Au Revoir."
If you're too timid to record your own greeting, there are two on YouTube which might work for you (see Resources).