How to know how much data I have left on iPhone?

How do I check how much data I have left, so that I can make sure not to overuse it.

Once you have wiped your phone for data, I think you should be safe.

Checking the amount of free memory in your iPhone

Pick the SETTINGS icon
Next menu, Pick "General"
General menu, pick "About"

From here scroll down to read the total memory and available memory.

Hope this helps