Converting mp3 and video content so I can listen to it on my iphone
iTunes plays both MP3 and AAC.

You can import one of theme and convert to the other by right-clicking on a song and selecting "create MP3 version" (or "create AAC version").

In iTunes you can set you can set the preferred format in the "import settings" (Preferences -> General -> Import Settings...).

Once they are in iTunes you can sync them on your iPhone or iPad.

For song found on YouTube you can google for "YouTube to MP3". You will find something (I'm not sure I can post a direct link here).