How do I activate my iMessages in iPhone 7 Plus?

My iMessages won’t activate. It just say waiting for activation. It’s been this way for a few days now. It also says send and receive to my email instead of my number. I’m new to the Apple world but it’s my understanding that so long as you and the person you are texting have iPhones, you can FaceTime and text for free without service. Well, my iMessages stopped working for some reason. Please help me!