How do I avoid getting charged for internet usage on my Blackjack II?

I just upgraded my AT&T phone to a Blackjack II because I desperately needed a new phone. I just wanted a cool looking phone (refurb was cheap) instead of the janky one I had, and I do not plan on purchasing a data plan anytime soon. I would purchase a plan in the future, but for now I'm good with just using it as a phone (I know it will be boring and useless, but I'm broke right now). What should I avoid doing or pushing that could eventually lead to me getting a shocking phone bill? Do I refrain from everything internet wise (aim, browsing, GPS, media net, wifi) or are any of these free to use?