If you're under thirteen years old, you can't use public chat. You have to use QuickChat, which is a feature provided by Runescape for communication. To use QuickChat, either click on the white speech bubble next to your name, or press enter. Otherwise, you have your Public Chat set to Off. It's the third button below the chat box; you can either set it to Off, Friends, Hide, or On. Set it to on to be able to hear yourself and everyone else.