Smart guitar in Garageband/iPad is great on the go, I'll try to point you in the right direction -
• The sideways guitar in 'Notes' mode is the same as a real guitar, from the bottom up EADGBE by string, but it depends on which fret you play. In chord mode, well, maybe a site like this would help you http://www.chordbook.com/guitarchords.php - it's basically just as though you're playing the right frets for each string by chord
• Chord mode is good if you want to figure out the approximate chords in a song or for rhythm, notes for more intricate/solo stuff
• autoplay should be fairly straightforwards, just means you don't have to pick strings, I'd say best used as rhythm
The app is very easy to get used to, just play with it!