Learn Spanish (Spain): Sécate el pelo con el secador.

Sécate is an affirmative command made from the verb secar (to dry) and the reflexive pronoun te. When you attach a pronoun to the end of a command, the spoken stress stays on the same syllable as the original verb (seca). Because adding te adds an extra syllable to the end of the word, Spanish spelling rules require a written accent on the é to keep the stress in the correct place.

Why does the sentence use el pelo instead of tu pelo for 'your hair'?

In Spanish, when talking about body parts or personal items, you usually use a definite article like el or la instead of a possessive adjective like tu (your). The reflexive pronoun te in sécate already makes it perfectly clear that it is your own hair being dried.

How can we tell this is a command for the informal tú rather than the formal usted?

There are two clues here. First, it uses the pronoun te, which always goes with tú. Second, it uses the informal affirmative command form seca. If you wanted to give this same instruction formally to an usted, you would change both the verb form and the pronoun, resulting in séquese.

Are the words secar and secador related?