When you play a song, the Now Playing screen appears:

Now Playing screen

Pause a song:
Tap or click the mic button on the iPhone headset.
Resume playback:
Tap or click the mic button on the iPhone headset.
Raise or lower the volume:
Drag the volume slider or use the buttons on the side of iPhone.
Restart a song or a chapter in an audiobook or podcast:
Tap .
Skip to the next or previous song or chapter in an audiobook or podcast:
Tap twice to skip to the previous song. Tap to skip to the next song, or click the mic button on the iPhone headset twice quickly.
Rewind or fast-forward:
Touch and hold or . The longer you hold the control, the faster the song rewinds or fast-forwards.
Return to the iPod browse lists:
Tap . Or swipe to the right over the album cover.
Return to the Now Playing screen:
Tap Now Playing.
Display a song’s lyrics:
Tap the album cover when playing a song. (Lyrics appear only if you’ve added them to the song using the song’s Info window in iTunes.)

You can display playback controls at any time when you’re listening to music and using another application—or even when iPhone is locked—by double-clicking the Home button.

If you’re using an application, the playback controls appear on top of the application. After using the controls, you can close them or tap Music to go to the Now Playing screen. If iPhone is locked, the controls appear onscreen, then disappear automatically after you finish using them.

Source: apple.com