There are several ways to talk with others in RotMG.
Public Chat This is the simplest way to talk to others. Simply press enter, type in your message, and press enter again. This type of message can be seen by anyone who is around you.
Private Chat When you want to talk with someone privately, this is the way to go. Type in "/tell examplename " or "/t examplename " before your message in order to privately message that person. You can also press the reply button (tab is the default) to automatically and quickly reply to the last person you sent a tell to.
Guild Chat If you are in a guild, you have the additional option of talking with your guild as a whole, by typing (without pressing enter) "g", or (After pressing enter) "/guild " or "/g " before your message. This lets you talk privately, but with more than a single person.
There are other ways of communicating with others, like the kabam board, the forums, IRC, voice chat servers, etc.