Why can't i receive text messages from people using iMessage when I have mine off?

I just got the iPhone 5c (same as the 5 basically) and when I have wifi I really do enjoy having iMessage (although I don't understand what is so different about it). I am really upset because when I go out and do not have wifi and i do not want to use my data (i only have 2 GB) for iMessage it does not run because i don't use my data which isnt a big deal but I cant receive texts from people who are using iMessage, only people that have normal texting. When I first got the phone (yesterday morning) it let me text people with imessage but without me having it but now i cant. im really upset because im out a lot and have no way to get in contact with people (almost all of the people i talk to have imessage) besides calling, which im 17 so i dont call that many people, its usually by text. i just really need help figuring this out its like stressing me out lol.

Any help at all would be so helpful.

Thank you!

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