How can I learn good words like saying Bismillah?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

You may dear brother, learn these good expressions by reading books that were written by scholars and in which they collected the authentic forms of dhikr that are  authentically reported from the Prophet (Sallallahu Alaihi wa Sallam).

One of the best of these books and the most suitable is the Hisn Al- Muslim written by Said Bin Wahf Al Qahtany. There is an English version of this book. This book is a compilation of the Adhkar (remembrance) of repentance and Tasbeeh (glorifying Allah) etc.

So, we recommend that you get a copy of this small book. In addition, we advise you to visit Islamic centers as often as possible and make acquaintances with Muslim brothers there and keep their company. They will- InshaAllah- help you greatly in following the right path.

Allah knows best.