You have a number of high-tech or low-tech ways to keep an eye on what your teen is doing online. One option is to make sure your teen uses his/her computer in a common area of the house, so you can literally peek over his/her shoulder. However, as so many teens have laptops, as well as multiple other places to access computers, this approach has limited utility. Some parents have insisted that their teens provide them with the passwords to relevant sites, such as a Facebook page or an email account. If you have your teen's passwords, you can access your teen's pages and see for yourself what's going on. The challenge with this approach is the toll it can take on your relationship with your teen. You might want to take the password approach with younger children and early teens. As you get confident that your growing teen can and is acting safely online, you can decide when your teen can create a truly confidential password.
Of course you do have a vast array of high-tech tools that can do the blocking and monitoring for you. The first type of software to consider actually blocks your teen's computer from reaching sites and content you might find objectionable. Such software isn't foolproof and might block sites that are in fact valuable for your teen to visit. The other type of software you can use is spying software, with different types spying in different ways. You can find software that records every key stroke, or sends you copies of emails and instant messages; there is even software that will help you monitor cell phone texts. You might even be able to use such software without your teen's knowledge. Again though, the challenge here is what your teen's discovery of your use of spyware will mean for your relationship.