Declaration of faith is to pronounce in words that there is no God but Allah and Prophet Muhammad is His messenger. The same should also be believed by heart. This is how to pronounce it in Arabic.
لَا إِلٰهَ إِلَّا الله مُحَمَّدٌ رَسُولُ الله (in Arabic)
There is no god but God, Muhammad is the messenger of God. (in English)