New 300MB data plan for an iPhone?

[Legion X]

I'm getting an iPhone and my parents got me the cheapest data plan available - 300MB a month.

About how much is that? I really only use Youtube, Twitter, and Facebook. Will any of those sites use up data faster than others?
Also, how can I check how much data I have left from my iPhone?
And lastly, I know that if I'm using a Wifi network I won't be using up space, but will I have to change the settings to make sure? Or does it switch to the Wifi network automatically?
Any other precautions I should take? Because I'm paying for the data plan and I really want to make sure I don't go over...

[redfive94901]

300MB is plenty for most people to do Twitter, Facebook, e-mail and surf a few times a day.
YouTube and any other streaming video (and audio too...Pandora; Spotify) will use up much more data, so be very careful with those.

Yes, you can check how much data you have been using both by logging into the account, depending on who your carrier is... and you can also check data Usage in the Settings > General > Usage.
You'll need to reset the counter on the day that a new plan-month starts. Please see link below.

If you save a specific Wifi network in the wifi settings, then the Iphone will connect to that wifi automatically. To be safe, you should watch the top notification bar and make sure it made the switch from 3G to wifi.