Use very clear language, fair and firm is really the way to go when giving your teen this large a privilege and responsibility. Create a parenting contract with your teen, I have a sample one here: Parenting Teens Contracts - Laptop Use. Feel free to add or delete parts of it to make it applicable to what you feel is important.
Be clear that there is no privacy when it comes to matter of email, social networking or blogging. While you will respect their space, your teen will have to respect your responsibility to them as their parent. There will be times you need to check up on their online activities. Your teen should comply with this or you will need to discontinue their use of the laptop.

