When the phone rings, you will see a green circle and a red circle. To answer, drag the green circle over the screen to answer. To reject, drag the red one.
If this is a hassle, you can also set it to use the home button at the bottom to answer calls. To set this up, go into phone (as if you were making a call) and bring up a menu by pressing the left hand corner button (next to the home button,) and select call settings from this pop up menu. Select Call answering/ending and select Answering Key. This will mean you press the home button to answer calls instead of swiping.
You can also set the power key to end calls here if this suits.