The Prophet's First Marriage
Youth is the period of the blooming of instincts and the emergence of one's sexual potencies. When youngsters, both male and female, are of mature age, they are drawn to the opposite sex, and a fire of passion starts flaming in their hearts that will not be extinguished unless they form a union of marriage. It is only in this way that they will find peace of mind.
Therefore, to make the proper use of such potentials and to prevent the various deviations that overflowing sexual instincts may create in human societies, Islam has emphatically ordered that the youth should marry as soon as possible and not shun the command of marriage on the pretext that they may be unable to support their family later on.
'And marry those among you who are single and those who are fit among your male slaves and your female slaves; if they are needy, God will make them free from want out of His grace; and God is Most Generous, Knowing. And let those who do not find the means to marry keep chaste until God makes them free from want out of His grace' (4:31-32).
But there may be times when financial conditions do not permit one to undertake the responsibilities of married life. No doubt, under such circumstances, marriage must be postponed until conditions are favourable, and, all through this period of celibacy, the youth must necessarily acquire virtue and chastity.
Muhammad suffered just such hard conditions. Due to financial problems, he was unable to take a wife until he was 25.  So he found it advisable to temporarily refrain from marriage and to wait for a suitable occasion when life's conditions would allow the formation of a family. 
Khadija's Business Proposal
Khadija, who was an honourable wealthy woman, used to put her wealth at the disposal of others who traded for her and received wages in return for their services.
As Muhammad's fame for honesty, virtue and trustworthiness spread throughout Arabia and reached Khadija, she started seeking his cooperation. Then she made this proposal to him: 'I will put at your disposal some property plus a servant, Masara, and pay you more than others'.
Being well aware of his uncle's financial problems due to his old age, low income, and large family, Muhammad accepted Khadija's ofher. 
Khadija, the daughter of Khuwalid, was a lady of supreme character. She had been twice married, to Abu Halah and Atigh Makhzumi, and twice widowed. Though she was forty years old, her enormous wealth, popularity, and prestige had led many wealthy and powerful Quraysh to court her.
But she did not accept any of them as her husband and avoided marriage, for she knew well that they either were interested in her wealth or were men whose character she detested. 
Prophet's Journey To Damascus
When the commercial caravan of the Quraysh was ready to start moving towards Damascus and the Prophet, too, had made provisions for the trip and was about to join the caravan, Khadija ordered her servant, Masara, to accompany him to Damascus and be always ready to serve him.
Obviously, it is not possible to explain in detail this historical journey, and we content ourselves with mentioning the following points: This journey brought about many blessings and much good fortune, such as enormous profits in commerce, the manifestation of the Prophet's wonderful personality to the people in the caravan, the meeting with the Christian monk, the prediction of his prophecy,  and the preliminary causes of an auspicious matrimonial union. When the trading was over, the caravan returned from Damascus.
Masara explained the trip to Khadija in detail, reporting the huge, unprecedented profits they had gained. She also spoke about the Prophet's excellent character and his generosity, as well as his many other virtues manifested during this journey. 
Upon hearing this and hearing about the predictions of a learned Jewish man about his divine character and his marriage with the most honorable woman of the Quraysh, Khadija not only started to cherish his love in her pure heart, but also came to realize that he was her ideal husband. 
Also, her uncle, Warqa ibn Nawfal, had talked to her about tile predictions of the last prophets, and about the good news of his marriage with Khadija.  These words, too, added to her love and enthusiasm.
But how was she to talk to him about her desire and heavenly affection? This was not so easy for Khadija, who was herself the most respected woman of the Quraysh.
Khadija's Proposal Of Marriage
Khadija asked Nafisa, who was her close friend and whom she always trusted with her secrets, to speak to the Prophet about marriage. Nafisa went to him and asked, 'Why do you not get married?' He answered, 'My living conditions and financial situation do not allow me to get married'.
Nafisa said, 'Will you agree to get married if this problem is solved and a rich, beautiful, and honorable woman from a well-known family asks you to marry her?'
He asked, 'Who is this woman you are talking about?' Nafisa answered, 'Khadija'.
He said, 'How is it possible? She has rejected the proposals of many of the Quraysh aristocrats and rich men. Would she marry me?' Nafisa said, 'This union is possible and I will arrange it'.
When he became quite sure of Khadija's inclination towards marriage with him, the Prophet talked to his uncles about the matter. They were very pleased with this good news, and they attempted to arrange the marriage for their blessed nephew. And finally this auspicious marriage was celebrated with special ceremonies. 
The Prophet spent 25 years of his life with Khadija, who was not only a loving wife for him, but also his best and most helpful mate.  This period is considered to be the best period of his married life.
Khadija, peace be upon her, was the first woman who believed in the Prophet's divine prophecy. She put all her wealth at his disposal to propagate and promote Islam.  Six children were born of his marriage: two sons named Qasim and Tahir who passed away as infants in Makkah and four daughers named Ruqiyah, Zaynab, Umm Kulsum, and Fatima, who was the most prominent and honoured of them all. 
Khadija was so devoted to her husband and showed such great sympathy and self-sacrifice for him and for the promotion of his religion that not only did he love her dearly and respect her highly during her lifetime, but even after her death. Each time he remembered her, his blessed heart filled with sorrow  and he wept at her loss. Khadija's brilliant sun of life set at the age of 65, ten years after the actualization of the prophetic mission of the Prophet.  In this way, the house of our Holy Prophet became deprived of the light of Khadija's existence forever.
. Bihar ul-Anwar, Vol. 16, p.3; Tarikh Ya'aqubi, Vol. 2, p.15.
. A'yan ul-Shi'ah, Vol. 2, p. s; Sirihi Halabiyih, Vol. 1, p.152.
. Sirihi ibn Hisham, Vol. 1, p.188; Bihar ul-Anwar, Vol. 16, p.22.
. Bihar ul-Anwar, Vol. 16, p.12; Tarikh Tabari, Vol. 3, p.1127.
. Sirihi ibn Hisham, Vol. 1, p.188. This monk was not the monk who met the Prophet in his childhood.
. Kamil ibn Athir, Vol. 2, p.39. Printed in Beirut, 1385 A.H.L.
. Bihar ul-Anwar, Vol. 16, pp.20-21.
. Sirihi Halabiyih, Vol. 1, p.152; Ayan ul-Shi'ah, Vol. 2, p.8.
. Bihar ul-Anwar, Vol. 16, pp.56-73.
. Ibid., pp. 7, 10; A'lam Alwari, p.146.
. Ibid., pp.10-71; A'yan ul-Shi'ah, Vol. 2, p.8.
. Ibid., p.3; A'yan, p.18; A'lam, p.146.
. Bihar ul-Anwar, Vol. 16, pp.8, 13.
Taken From: A Glance at the Life of the Holy Prophet of Islam , by Dar Rah Haqq's Board of Writers , Translated by: N. Tawheedi