As Dolenz said, some of those 'A' notes are different (low , middle and high - and even one which is very high). Which A are you asking about? I guess, if you are a beginner, it's the one on the 2nd space of the staff - (the one above the G line that the treble clef curls around).
That particular A can be found in four places on the guitar. It can be played on string 3 fret 2, string 4 fret 7, string 5 fret 12 and string 6 fret 17.
But as a beginner, you can forget about ALL of them except the first one - string 3 fret 2. A beginner's book on reading music for guitar will introduce that one on string 3 long before you ever get to learn about its alternative location on string 4 or elsewhere.
The other two As that you will learn as a beginner are the high A on string 1 fret 5 and the low A on the fifth open string. Those also have alternative locations, but it will be ages before you ever have to worry about those.
When you're more advanced, a lot of the time, it will be up to you to decide which is the best place (i.e., most sensible place) to play any particular note. As Dolenz also wisely points out, it depends what your other fingers are doing at the time. If you are already up near the twelfth fret, it would be easier to find a middle A up there somewhere than come charging all the way down to the second fret just for one note.
If the songwriter, composer or arranger wants a note played in a particular place they will show that by placing the string number inside a circle, placed next to the note, or by telling you which fret position to place your hand (they use Roman numerals for that) - otherwise it's up to you. Just don't confuse a low A with a middle A or a high A
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