PhD students are engaging more and more in science communication. Some students are motivated by the challenge of conveying their science in a different way than what they see in the office or the lab, while others are motivated by spending some time outside the traditional scientific program.
Either way, it has plenty of benefits. Not only for the student, but also in the public space. PhD students are typically younger than the senior researchers and this can work as an inspiration for young school children and high school teenagers.
A simple way to get started with science communication as a PhD student is to bring a phone with you to the lab or in the office and simply photograph and share what you work on, how the process is, what obstacles do you meet, which parts do you like/dislike and so on. Share in your network via social media (Instagram is excellent for this) and develop your communication from there.
Ask at your local communication office if they have some guidelines. They can help you do outreach on behalf on the university and help you advertise.