1. Set some rules and expectations. Like they're not allowed to use it after 9:00 PM. Or they can't watch videos, because they'll blow through all your data. Whatever the rules are, make sure you both agree to them beforehand.
You also might want to get their passwords to all their social media accounts. If they want their own phone, that's the deal they have to agree to.
2. Talk to them about sexting. Even if they forward a picture of someone else, they can get in real trouble for it. So explain why it's a big deal. They'll probably roll their eyes, but you have to talk about it.
3. Talk to them about cyberbullying. Explain what it is, and make them promise to tell you if someone starts bullying them online. Also make them promise not to meet up with any STRANGERS they meet online.
4. Make sure they realize it's a big responsibility. Phones are expensive. And you're also trusting them to make good decisions.
5. Make sure they know it's really YOUR phone. You paid for it, and you can take it back whenever you want. So they have to follow the rules.