What dua do I say after wudu?

The main dua

Transliteration: Ash-hadu an la ilaha illallahu wahdahu la sharika lah, wa ash-hadu anna Muhammadan 'abduhu wa rasuluh.

Meaning: "I bear witness that there is no god but Allah alone, with no partner, and I bear witness that Muhammad is His servant and Messenger."

This is reported in Sahih Muslim 234. It is basically the shahada you said when you became Muslim, so you already know most of it. Every wudu becomes a small renewal of that moment.

The addition about purity

Another narration adds: "Allahumma-j'alni minat-tawwabina waj'alni minal-mutatahhirin", meaning "O Allah, make me among those who repent often and make me among those who purify themselves" (Jami' at-Tirmidhi 55; some scholars grade it weak). It echoes the Quran:

Some scholars also mention saying "Subhanakallahumma wa bihamdik, ash-hadu an la ilaha illa ant, astaghfiruka wa atubu ilayk" (Glory be to You, O Allah, and praise; I bear witness there is no god but You; I seek Your forgiveness and turn to You), which is reported from the Prophet ﷺ in other collections.

What about duas for each body part?

You may see lists with a special dua while washing the face, the arms and so on. Scholars of hadith say these are not authentically reported from the Prophet ﷺ, so you don't need to learn them. The sunnah is "Bismillah" at the start and the shahada at the end. Between those, you can stay quiet or remember Allah in your own words.

Making wudu in a bathroom is normal. Many scholars say to say the words quietly or in your heart if you are inside a toilet room. See What dua do I say when entering the bathroom?

How to remember it

Write it on a sticky note by the sink, or save it on your phone until it sticks. See Can I read my duas from my phone or a paper? You can also say it facing any direction; facing the qibla is nice but not required.