If you are a beginner, just starting out learning to play the guitar, your fingertips are likely to be soft. When you fret a single note or a chord, the string will sink into the flesh of your fingers. When this happens, another part of the finger may be contacting another string.

As you develop calluses, this occurrence is much less likely as the calluses develop, the string will not sink into the flesh anywhere near as much. Another possibility is that the fingers are not appropriately angled as they contact the strings.

If they are leaning over rather than straight up and down, they will touch an adjacent string. If all else fails, you may need another guitar that offers a wider fretboard. This is not very common but may need to be considered.

Before taking such drastic action, consult a professional guitar teacher who will confirm if your fretting hand and fingers are correctly positioned.