The following notation and diagram show all the notes in the Open Position on the guitar. The term "open position" means the open string notes and the notes on the first three frets.
As you play through the notes you will notice B to C and E to F are only one fret apart (a semitone), whereas all other notes are two frets apart (a tone). You will learn more about tones and semitones in lesson 5. For now, the main thing is to remember where they occur. You can learn them by picturing a musical alphabet set out as follows: