If you're using OS X Mountain Lion iTunes is available for syncing. Learn about that and other OS X Mountain Lion entertainment features here.
iTunes allows you to group the songs in your library into playlists so you can have different mixes for different situations (such as exercising or hanging out with friends). If you want, you can also create playlists of videos.
You can also create a playlist by selecting one or more files in your library and navigating to File > New > Playlist from Selection.