I Took the Plunge and Switched to an iPhone 5c

I found out a few months ago that I was going to be eligible for an upgrade on my phone this month.  I had been wanting to get my hands on an iPhone ever since and so Monday night, I took the plunge.

I had a Blackberry for a few years and I realized that was meant for someone who is more organized than me.  Then I switched to an Android and I really liked it, but I have friends who swear by their iPhones.

So I gave in to peer pressure, went in to the store and reviewed my options for an iPhone. I could have gone with an older model or the latest iPhone 5s.  I decided the 5c would be the best for me because it was a little cheaper, and since I'm an iPhone virgin, I figured this would be a good one to start with.

The only predicament I'm having right now is just learning all the different features of the phone.  I need a little help -- what is the one thing I should know about the iPhone or an essential app I should download on it?