King Bharat, the ruler of Hastinapur had nine sons. When it was time to choose a heir he was hesitated and finally decided to make none of them the heir of throne. He implemented his belief of "Karma" above " Birth". And Shantanu was made the King.
King Shantanu fell in love with Devi Ganga. He asks her to marry him, to which Ganga agrees but with only one condition, that "King Shantanu should never ask Devi Ganga any question, if he will ask her, she will leave him forever". The pink heart of King promptly agrees with the Devi. After their marriage Devi Ganga gives birth to a son, heir to the throne of Hastinapur. Devi Ganga takes the newborn child and to disbelief of King Shantanu drowns him in the river. King Shantanu is astounded and hurt, but to his condition( VACHAN), he couldn't ask her. And the same scenario repeated for more 6 time. Thus in total 7 of the new born child of King Shantanu and Devi Ganga are drowned by Devi in the river. When their 8th child is born, King Shantanu finally made the courage and stopped Devi from drowning the child.
Devi Ganga answers King Shantanu that she has actually freed their newborns from a curse. She is actually from heaven and is daughter of Brahma. And on earth she is living a curse along with King Shantanu. In the previous life King Shantanu was King Mahabhishak, who was also a friend of Lord Indra. he usually used to visit Lord Indra to watch the dance of celestial damsels. One day Lord Brahma also visited the court along with his daughter Ganga. King Mahabhishak and Devi Ganga looked at each other and instantly were infatuated with each other. They looked in each others eye without blinking. At the moment a light breeze blew away the stole( aanchal/dupatta) of Devi Ganga. Because of that everyone averted their gaze from the damsels and looked away as a form of respect, but King Mahabhisak lost in the eyes of Devi Ganga didn't notice the event and kept looking in her eyes. This act was was seen by Lord Brahma. Lord Brahma was infuriated and cursed King Mahabhishak and also Devi Ganga as she was a willing participant in the digressing event, that in their next life both of them will be born on Earth(Mrutyu Lok).
To free their newborns from the curse i.e to live on earth, Devi Ganga drowns them in the river. And since King Shantanu has broken his condition (vachan) given to Devi Ganga, she is also freed from the curse.She takes the new born child who she names, "DEVRATA" with her.
In the hope that Devi Ganga would meet him someday, he would usually take a stroll by the river. One day he finds two new born children by the river, as there was no one around and they happened to be children of some sage, he takes them with him. As they were found by grace of god, he names the boy child, "KRIPA" and the girl child "KRIPI". These children were given to the royal priest for their bringing them up. Kripi later grows up to become the wife of DRONOACHARYA.
When the right time comes she comes back to earth along with Devrata and gives him to King Shantanu. Devrata has excelled in his skills of archery which he has learned from Rishi Bhargava. Rejoiced by the return of his son, he names him the Yuvraj of Hastinapur.
One day King Shantanu and Yuvraj Devrata go for hunting. On their course, King Shantanu sees a beautiful lady in a boat. She turns out to be SATYAVATI, she helps passerby to cross the river. King Shantanu though being rejoiced with the return of his son, still felt a void in his heart, void of not having a loved one. To ask Satyavati's hand for marriage King Shantanu visits her house. "Dasa", her father readily accepts the marriage but puts a condition in front of the king, that the offspring of Satyavati and Shantanu should only be made heir to the throne. Since King Shantanu has already given his word to Devrata, he defies the condition and storms out of the house. When Devrata comes to know about his father's love with Satyavati, he himself goes to the house of Satyavati and convinces her to marry his father King Shanatanu. On realizing the complexity of the scenario he takes a oath (PRAN) that he would never sit on the throne and he also vows(PRATIGYA) celibacy. Thus he unites his father King Shantanu with Satyavati. For his act King Shantanau pronounces Devrata as "BHISMA". And King Shantanau also gives Devrata a boon (Vardaan), that he can not die, till he wishes to die, "IchhaMrutyu".
King Shantanu has two sons with Satyavati, Chitrangada and VichatraVir. After the demise of King Shantanu, Chitrangada sits on the throne but within few days, he passes away. And then Vichatravir rules Hastinapur. Time passes and Vichitravir reaches the age of marriage. Kashi and Hastinapur had old relations between them since, princess of Kashi was married to King Bharat of Hastinapur. But the relations got soured when King Shantanu refused the marriage proposal of princess of Kashi with Devrata. So for the swayamvar of princesses of Kashi, Amba, 
On meeting Satyavati, Amba confesses to her she was in love with King of Shaalva and in the swayamvar she already had chosen her husband. Satyavati and Bhisma, heartily accepted her dialogue and allowed her to go to King of Shaalva . When the king sees Amba, he feels insulted as he thinks he was given Amba out of pity and instead there should have been a duel or war between him and Bhisma, and so he doesn't accepts Amba and sends her back to Bhisma. Amba takes this a grave disrespect. She feels she is disdained because of Bhisma, if only Bhisma wouldn't have sent back her to Shaalva, then nothing would have happened. To rectify the mistake, she demanded Bhisma to marry her. But since Bhisma has taken the vow of celibacy, he denies her proposition. Amba gets deeply hurt by the words of Bhisma and takes vow in front of the court of Hastinapur, that she would never forgive Bhisma and would avenge her insult by becoming the reason of Bhisma's death. And she storms out of the court.