How to keep "Messages" from Opening Automatically

How do I keep "Messages" from Opening Automatically.

As soon as I turn my computer on, or open it back up, after a few seconds, "Messages" pops up even if I don't have any messages. I only use it as a Text Messaging program. Basically so I can text other iPhone users without using my phone all the time.

It is extremely annoying. I've searched through all of the settings within the program preferences and OSX System Preferences. But I haven't been ablt to find anything helpful to keep it from opening.

Thanks