Start by thinking about your scalp rather than only your hair length. An oily scalp may need a cleansing formula used regularly, while a dry or easily irritated scalp may feel better with a gentler option. Hair that is coloured, curly or frequently styled may need extra conditioning support.
Read the product label and avoid choosing only because a product is popular. Notice whether the formula is intended for your main concern, such as dryness, buildup or frizz. You may also need a separate conditioner for the lengths of your hair.
Use a small amount and massage it into the scalp instead of scrubbing the hair lengths aggressively. Rinse thoroughly, and adjust how often you wash according to your scalp's needs, weather and activity level.