So are you on the 150MB/mo data plan then? Well, whether it's 150 or 300 or whatever you will almost certainly exceed it greatly with regular usage
unless you actively take proper steps to limit your use. And since you don't seem to know what this is you're in danger, so I hope you can learn quickly
The "data plan" you pay for is called 3G, which is your phone connecting wirelessly to the nearest cell tower to send info to and receive info from (typically lots this way) it. The other ways your phone can typically exchange info are:
1X - the regular wireless connection to cell towers, mostly for phone calls and SMS (text) messages
WiFi - a wireless ethernet connection, typically set up mostly for laptop computers and such to use
USB cable - connect directly to a computer.
On a limited plan, I strongly suggest you turn 3G off on your phone except for relatively rare occasions when you are out and about and really need to access email or look something up quickly. When you want to load stuff like songs or video or some new app to your phone, make sure you are on Wifi - could also use a USB cable but that's more of a pain. Running apps or playing media already on your phone won't "use data" aka Talk to the Outside World. However note that many apps do access the internet so will use some (or lots of) data, but if 3G is turned off on your phone they will only be able to if you have wifi access (which Verizon doesn't charge you for data passing here).