What did Prophet Muhammad do for a living before he became a prophet? Please tell me what he did to gain money or to help out.
Well not only the seerah books but also the hadith compilations quote two major occupations:
Narrated Abu Huraira: The Prophet (ﷺ) said, "Allah did not send any prophet but shepherded sheep." His companions asked him, "Did you do the same?" The Prophet (ﷺ) replied, "Yes, I used to shepherd the sheep of the people of Mecca for some Qirats." (Sahih al-Bukhari)
(which apparently is not approved by shi'a according wikipedia)
In the age of 12 he accompanied his uncle for trading to Syria (a-Shaam):
When the Messenger of Allâh (Peace be upon him) was twelve years old, he went with his uncle Abu Talib on a business journey to Syria. When they reached Busra (which was a part of Syria, in the vicinity of Howran under the Roman domain) they met a monk called Bahira (his real name was Georges), who showed great kindness, and entertained them lavishly. He had never been in the habit of receiving or entertaining them before. He readily enough recognized the Prophet (Peace be upon him) and said while taking his hand: “This is the master of all humans. Allâh will send him with a Message which will be a mercy to all beings.” Abu Talib asked: “How do you know that?” He replied: “When you appeared from the direction of ‘Aqabah, all stones and trees prostrated themselves, which they never do except for a Prophet. I can recognize him also by the seal of Prophethood which is below his shoulder, like an apple. We have got to learn this from our books.” He also asked Abu Talib to send the boy back to Makkah and not to take him to Syria for fear of the Jews. Abu Talib obeyed and sent him back to Makkah with some of his men servants. (Ar-Raheeq al-Makhtum, Chapter on Bahira, the Monk, see for example online page 34 here)
He also was known as the trustwhorty (al-Ameen الأمين), so many people trusted him their goods.
And he toke part in harb al-Fijar (or Sacrilegious Wars) a war between Kinanah (Quraish is part of this tribe) and Qais 'Ailaan as stated by ibn Sa'ad in his tabqaat. But he didn't raise arms against the opponents according al-Mubarkpuri in his ar-raheeq al-Makhtum:
His efforts were confined to picking up the arrows of the enemy as they fell, and handing them over to his uncles. (Same source as before on the same page)
Narrated Ibn 'Umar (ra) : We were accompanying the Prophet (ﷺ) on a journey and I was riding an unmanageable camel belonging to 'Umar (ra), and I could not bring it under my control. So, it used to go ahead of the party and 'Umar would check it and force it to retreat, and again it went ahead and again 'Umar forced it to retreat. The Prophet (ﷺ) asked 'Umar to sell that camel to him. 'Umar replied, "It is for you O Allah's Messenger !" Allah's Messenger (ﷺ) told 'Umar to sell that camel to him (not to give it as gift). So, 'Umar sold it to Allah's Messenger (ﷺ). Then the Prophet (ﷺ) said to 'Abdullah bin 'Umar "This camel is for you O 'Abdullah (as a present) and you could do with it whatever you like." (Sahih al-Bukhari)
This is one out of many ahadith showing that the Prophet () even kept trading (either) by himself (or via intermediate or by hiring) after the first revelation.
Al-Mubrakpuri says in his ar-Raheeq al-Makhtum speaking about Muhammad's Early Job:
Muhammad (Peace be upon him), had no particular job at his early youth, but it was reported that he worked as a shepherd for Bani Sa‘d and in Makkah. At the age of 25, he went to Syria as a merchant for Khadijah (May Allah be pleased with her) Ibn Ishaq reported that Khadijah, daughter of Khwailid was a business-woman of great honour and fortune. She used to employ men to do her business for a certain percentage of the profits. Quraish people were mostly tradespeople, so when Khadijah was informed of Muhammad (Peace be upon him), his truthful words, great honesty and kind manners, she sent for him. She offered him money to go to Syria and do her business, and she would give him a higher rate than the others. She would also send her hireling, Maisarah, with him. He agreed and went with her servant to Syria for trade. (same online source page 35)
This clearly says that Muhammad was hired to carry merchandise for Khadija. read Also in the above mentioned biography of the Prophet by ibn Ishaaq the pages 82 ff.
In the name of Allah, the most compassionate, the most merciful
Based on my research, Prophet Muhammad (S) passed his period of teenage time in his uncle’s house (Abu-Talib), and was active in commercial affairs as well as helping his uncle. It is remarked that the Prophet has been commercial partner associated with Saaeb ibn Abi-Saaeb, and were commercial affairs to each other.
On the other hand, based on what I figured out in regards to the occupation of shepherd, seemingly, Sunnis agree that the/a job of Prophet Muhammad was shepherd, but apparently Shia Islam doesn’t believe so, albeit there appear to be hadiths which point out that shepherd was regarded as a/the job of the Prophet, but Shias have discussion about the authenticity of such narrations … Meanwhile, there doesn’t appear to be an explicit verse in the Quran to mention it, too.
As a related helpful context:
The Prophet’s grandfather, ‘Abdu ‘l-Muttalib, was the chief of Banu Hashim and also the guardian of the Ka’bah. His father was called ‘Abdullah and his mother, Aminah bint Wahab. His father passed away a few months before his birth. At the age of six, the Prophet lost his mother as well and was placed under the care of his paternal grandfather, ‘Abdul Muttalib. But his grandfather also passed away after four years; and at this time the Prophet’s uncle, Abu Tālib, took charge of him and became his guardian, taking him to his own house. Thus the Prophet grew up in his uncle’s house and even before reaching the age of adolescence used to accompany his uncle on business journeys by caravan.
The Prophet had not received any schooling; yet, after reaching the age of maturity he became famous for his wisdom, courtesy, trust-worthiness and truthfulness. He soon became famous as “as-sādiq al-amīn” (the truthful, the trustworthy). Abu Tālib used to say: “We have never heard any lies from Muhammad, nor seen him misconduct himself or make mischief. He never laughs unduly nor talks untimely.”
As a result of his sagacity and trustworthiness, Khadija bint Khuwaylid, a Qurayshi lady wellknown for her wealth, appointed him as the custodian of her possessions and left in his hands the task of conducting her commercial affairs. The Prophet once journeyed to Damascus with Khadija’s merchandise and as a result of the ability he displayed was able to make an outstanding profit. Before long she asked to become his wife and the Prophet accepted her proposal. After the marriage, which occurred when he was twentyfive years old, the Prophet began the life of a manager of his wife’s fortunes, until the age of forty, gaining meanwhile a widespread reputation for wisdom and trustworthiness.
Source and further information:
You can easily find out in the various books written on the life of Prophet Muhammad (PBUH). In his boyhood he used to herd sheep and goats. And in his young age he went in business travels with his uncle Abu Talib. And after that he also traveled by his own in different business Caravans, selling clothes and different goods in different markets.
I will try to find some references and add them to my answer later.