You can either text through your iPhone, which requires your phone to be on and connected to your carrier's network, as well as have a data connection, or you can use a texting app. Hangouts/Google Voice is the best option as it really is a traditional phone/text replacement. You get your own number that people can call and text for free, and it's the only service of its kind, as far as I know, that can do MMS (picture/video messages and group chats). That's what I use and save $45/month on my phone bill.