Learn Portuguese (Portugal): Tu vestes-te rápido.

Vestir normally means "to dress" (like dressing someone else). To say "to get dressed" (to dress yourself), Portuguese uses the reflexive verb vestir-se. Because the subject is tu (you), the reflexive pronoun changes to -te (yourself).

Why is the pronoun -te placed after the verb instead of before it?

In European Portuguese, reflexive pronouns usually attach to the end of the verb with a hyphen in simple, positive sentences. So we write vestes-te rather than te vestes (which is more common in Brazilian Portuguese).

Is rápido an adjective or an adverb here?

Rápido is technically an adjective (meaning "fast" or "quick"), but in everyday Portuguese it is very often used as an adverb to mean "quickly". You could also use the actual adverb rapidamente, but rápido sounds much more natural and casual.

How is the verb vestir-se conjugated here?

Vestir-se is an -ir verb. For tu, you drop the -ir and add the regular ending -es, giving you vestes. Just watch out for the eu (I) form, which has a spelling change: eu visto-me.