I have a question about AT&T plans?

Okay. Im sixteen years old and I have to buy my own phone for the first time. My old contract ended in November and I was waiting to sign a new contract until I needed a new phone. Im looking at the AT&T ANDROID phones, but I'm not sure which one is best so if you have any opinions on those phones please share. But my main question is that it says the phone costs 49.99 with a two year contract..but what does a two year contract mean exactly? Am I gonna have to pay extra each month? Please give me any information you possess it would help me a lot. I know with the Iphone you have to pay 30 dollars extra each month am I going to have to do that with the Android phone? Please and Thank You.