1) What's more, parents often worry about the effect of technology on their teenager's physical and mental health.
2) However, teenagers spend too much time at the screen, they can find bedtime a struggle.
3) To sum up, be aware of cyber-bullying that happens on the Net. If you face such a problem, discuss it with your parents.
4) Moreover, it is important to protect personal information and it's more important to develop real life relationships. Be sure that you don't spend more time online than making real friends.
Ответ: more, However, sum up, Moreover.