Did you know Bismillah was in the first revelation of the Quran?
The Quran’s very first revelation, “Iqra bismi rab bikal lazee khalaq”, contained bismillah. In this context, bismillah followed a verb, indicating that it was associated with a specific action – the revelation of the Quran.
Glorifying Allah (subhanahu wa ta’ala) with Bismillah:
In Bismillah, we’re calling upon the name of Allah (subhanahu wa ta’ala) and not Allah (subhanahu wa ta’ala) directly. This is because the Quran teaches us to make tasbeeh to Allah (sabbahalillah) and to the name of Allah (sabbihismarabbik), to make tabarak to Allah (tabarakallahu) and to the name of Allah (tabarakasmurabbik), to remember Allah (uzkurullah) and the name of Allah (wazkurismarabbik). Hence, the Quran tells us to do certain things sometimes by mentioning Allah (subhanahu wa ta’ala) directly and sometimes by mentioning only the name of Allah (subhanahu wa ta’ala).
This shows how special and holy the name of Allah (subhanahu wa ta’ala) is. By honoring only the name of Allah (subhanahu wa ta’ala), we’re expressing how much more respect and glory Allah (subhanahu wa ta’ala) truly deserves.
Remember, we glorify and exalt Allah (subhanahu wa ta’ala) and the name of Allah (subhanahu wa ta’ala), but we worship only Allah (subhanahu wa ta’ala). In other words, we make tasbeeh, tabarak to Allah (subhanahu wa ta’ala) and his name, but we worship only Allah (subhanahu wa ta’ala).
When to say Bismillah:
Bismillah for every deed
Basmala (reciting Bismillah) is recommended before starting any action or deed. In a famous Hadith Prophet Muhammad (sallallahu alaihi wasallam) said, every single deed of importance that does not begin with Bismillah, it is as if the deed was amputated/mutilated(cut-off).

After Isti’adhah
Bismillah-ir-rahman-ir-rahim is recited after Isti’adhah because after we ask Allah (subhanahu wa ta’ala) for protection from bad external threats we ask Allah (subhanahu wa ta’ala) to bless us with right internal thoughts. It could be thought of as A’oodhubillah and Bismillah work in cooperation with each other. We seek refuge in Allah (subhanahu wa ta’ala) from bad and then ask Allah (subhanahu wa ta’ala) for the good.

To curse Shaytan
In a Hadith, Prophet Muhammad (sallallahu alaihi wasallam) mentioned that even if you want to curse Shaytan, don’t do it because it makes Shaytan feel bigger(bigger than a house) as he is causing the issue and you are remembering him. Instead, saying Bismillah will cause Shaytan to shrink to the smallest possible size(for eg: size of a fly). Hence mentioning the name of Allah (subhanahu wa ta’ala) will bring blessings/barakah of Allah (subhanahu wa ta’ala).

Before Khutbah
Basmala is recommended before starting a Khutbah (speech).

Hadith from Sunan Abi Dawud
Abu al-Malih (radi allahu anhu) reported on the authority of a man: I was riding on a mount behind the prophet (sallallahu alaihi wasallam). It stumbled. Thereupon I said: May the devil perish! He said: do not say; may the devil perish! For you say that, he will swell so much so that he will be like a house, and say: by my power. But say: in the name of Allah (subhanahu wa ta’ala); for when you say that, he will diminish so much so that he will be like a fly.
Before ablution
Basmala is recommended at the beginning of ablution

Before entering the restroom
Basmala is recommended before one enters the place where he wants to relieve himself.

Hadith from Sunan Abi Dawud
Narrated AbuHurayrah (radi allahu anhu): The Messenger of Allah (sallallahu alaihi wasallam) said: The prayer of a person who does not perform ablution is not valid, and the ablution of a person who does not mention the name of Allah (subhanahu wa ta’ala) (in the beginning) is not valid.
Hadith from Jami’ at-Tirmidhi and Sunan Ibn Majah
Ali bin Abi Talid (radi allahu anhu) narrated that: The Messenger of Allah (sallallahu alaihi wasallam) said: “The screen between the eyes of the jinns and nakedness of the children of Adam when one of you enters the area of relieving oneself is saying: ‘Bismillah‘”.
Before eating
Basmala is recommended before eating. Prophet Muhammad (sallallahu alaihi wasallam) said to Umar bin Abi Salamah while he was a child under his care – say Bismillah, eat with your right hand, and eat from whatever is next to you. Some of the scholars have stated that Basmala before eating is obligatory.

Before sexual intercourse
Basmala is also recommended before having sexual intercourse. The Two Sahihs recorded that Ibn Abbas (radi allahu anhu) said that the Messenger of Allah (sallallahu alaihi wasallam) said, if anyone of you before having sexual relations with his wife says, ‘In the Name of Allah (subhanahu wa ta’ala). O Allah (subhanahu wa ta’ala)! Protect us from Shaytan and also protect what you grant us (meaning the coming offspring) from Shaytan’, and if it is destined that they should have a child, then, Shaytan will never be able to harm that child.

Hadith from Sahih Al-Bukhari and Sahih Muslim
‘Umar bin Abu Salamah (radi allahu anhu) reported: I was a boy under the care of Messenger of Allah (sallallahu alaihi wasallam), and my hand would wander about in the dish. Messenger of Allah (sallallahu alaihi wasallam) said to me “Mention Allah’s (subhanahu wa ta’ala) Name (i.e., say Bismillah before you start eating), eat with your right hand and eat from what is near to you”. I always followed this way of eating after this incident.
Hadith from Sahih Al-Bukhari and Sahih Muslim
Ibn ‘Abbas (radi allahu anhu) reported: The Prophet (sallallahu alaihi wasallam) said,”If anyone intends to have (sexual intercourse) with his wife, he should say: “Bismillah! Allahumma jannibnash-Shaitana, wa jannibish-Shaitana ma razaqtana (In the Name of Allah (subhanahu wa ta’ala), O Allah (subhanahu wa ta’ala)! Keep us away from Satan and keep Satan away from what you have bestowed upon us);’ and if Allah (subhanahu wa ta’ala) has ordained a child for them, Satan will never harm him.”