It's just alternating between E3 and F3 and increasing in from whole notes to half notes to quarter notes to 8th notes to 16th notes and so on, with each change being after four measures of the other. It'd be D string, second fret for E3 and D string, third fret for F3. The notes on the bottom of the line (assuming this is the treble clef) are E3, and the note above that between the two bottom lines is F3.