New to the world on iPhones. Tips on battery life and data usage?

Hi, I just got my first iPhone recently, and was wondering if anyone had any suggestions for me. I got a case for it, so we're good there.

I was mostly wondering if there's anything I need to do so that while I'm not on my home internet I don't receive a lot of data or whatever to make me go over my data limit (which is 300 MB a month)

Also, is there anything special I should know about battery-life? Seems different people say different things, like you need to let your phone be almost dead before charging it, or, oppositely, you need to always charge your phone to ensure good battery life, etc.

Also, someone was telling me texting, videos and pictures sent between iPhones, no matter what carrier, is free. Is this true? Any restriction apply that I should know about?

Thanks so much for all your help!
God bless~ :D

Turn on imessage in your settings, its probably on by default. For battery the rule is turn off things you don't need. Dim the screen some, don't need it full brightness (mine is around 60%). See this: http://www.gatorspit.com/tips/iphone-battery-save.html
I also turn off background updating, no reason to always have that checking, I'll update apps if I want manually… The more automated stuff you have running like that when you don't need it then the more battery it takes.