How to tell how much data I have left on my iPhone?

Ok I'm a first time smart phone user and I just got an iPhone (yay!) my dad told me I have a 4GB data plan (which I'll mostly use for safari) I looked on settings and went on usage and it showed like sent and received. Which is which? Is sent how much I've used and is received how much I have left? I don't wanna run my parents over if i use my iphone somewhere! ***Also! If I use my house's wifi on my iphone will it use up my data? I just want to know so i don't have to lug around my ipod touch as well.

I recently got an iPhone 4s and went through the same dilemma (except I only have 1GB of Internet Data).To answer your first question, the amount of internet data that you use is determined by adding BOTH numbers - received and sent. And you should keep in mind that 1000 mb of used data is roughly the same as 1 GB, so you can use approximately 6000 mb.
Also, if you use your house's wifi, it will not use up your data. Since you aren't receiving or sending data from your carrier, none of your data would be used.