Understanding left-hand fingering in guitar tablature (or just tab) is not as hard as you might think. Most of the time, there won't be any notation in the tab to alert you to which fingers to use because you most often play in position.
A position on the guitar is a group of four consecutive frets; for example, frets 1, 2, 3, 4 or 5, 6, 7, 8. The first fret in a series of four marks the beginning of a new position; for example, frets 2, 3, 4, and 5, frets 3, 4, 5, and 6, and so on, are positions as well.
The easiest way to play melodies on the guitar is to play them in first or second position — that is, using frets 1 through 4 or frets 2 through 5. These positions are close to the nut, allowing you to easily and smoothly use both the open strings and the fretted notes in a melody.
Open position itself consists of the combination of all the open strings plus the notes in the first or second position. The chords that you play low on the neck using open strings (A, D, Em, and so on) are known as open chords.
In any position, each finger plays the notes of a specific fret — and only of that fret. In first position, for example, the first finger (the index finger) plays the notes in the first fret; the second finger (middle finger) plays the notes in the second fret; and so on. Using one finger per fret enables you to switch between notes quickly.