How can one get his sins especially major sins forgiven?
Repent, pray and ask for forgiveness. Allah is kind. Leave it upto Him, just do your part.
Just seconding the above post of 5Abi, I am posting a few of Hadith Qudsi relevant to your query, insha'Allah these beautiful and heart-touching words of Allah (swt) conveyed on the blessed tongue of Rasool Allah (saw) will encourage us all to seek forgiveness for our slip ups.
On the authority of Abu Harayrah (may Allah be pleased with him), who said that the Prophet (PBUH) said: Allah the Almighty said:
I am as My servant thinks I am (1). I am with him when he makes mention of Me. If he makes mention of Me to himself, I make mention of him to Myself; and if he makes mention of Me in an assembly, I make mention of him in an assemble better than it. And if he draws near to Me an arm's length, I draw near to him a fathom's length. And if he comes to Me walking, I go to him at speed.
(1) Another possible rendering of the Arabic is: "I am as My servant expects Me to be". The meaning is that forgiveness and acceptance of repentance by the Almighty is subject to His servant truly believing that He is forgiving and merciful. However, not to accompany such belief with right action would be to mock the Almighty.
It was related by al-Buhkari (also by Muslim, at-Tirmidhi and Ibn-Majah).
On the authority of Abu Hurayrah (may Allah be pleased with him), who said that the Messenger of Allah (PBUH) said:
A man sinned greatly against himself, and when death came to him he charged his sons, saying: When I have died, burn me, then crush me and scatter [my ashes] into the sea, for, by Allah, if my Lord takes possession of me, He will punish me in a manner in which He has punished no one [else]. So they did that to him. Then He said to the earth: Produce what you have taken-and there he was! And He said to him: What induced you to do what you did? He said: Being afraid of You, O my Lord (or he said: Being frightened of You) and because of that He forgave him.
It was related by Muslim (also by al-Bukhari, an-Nasa'i and Ibn Majah).
On the authority of Abu Hurayrah (may Allah be pleased with him) that the Prophet (PBUH), from among the things he reports from his Lord (mighty and sublime be He), is that he said:
A servant [of Allah's] committed a sin and said: O Allah, forgive me my sin. And He (glorified and exalted be He) said: My servant has committed a sin and has known that he has a Lord who forgives sins and punishes for them. Then he sinned again and said: O Lord, forgive me my sin. And He (glorified and exalted be He) said: My servant has committed a sin and has known that he has a Lord who forgives sins and punishes for them. Then he sinned again and said: O Lord, forgive me my sin. And He (glorified and exalted be He) said: My servant has committed a sin and has known that he has a Lord who forgives sins and punishes for sins. Do what you wish, for I have forgiven you.
It was related by Muslim (also by al-Bukhari).
On the authority of Abu Hurayrah (may Allah be pleased with him), who said that the Messenger of Allah (PBUH) said:
Our Lord (glorified and exalted be He) descends each night to the earth's sky when there remains the final third of the night, and He says: Who is saying a prayer to Me that I may answer it? Who is asking something of Me that I may give it him? Who is asking forgiveness of Me that I may forgive him?
It was related by al-Bukhari (also by Muslim, Malik, at-Tirmidhi and Abu Dawud).
In a version by Muslim the Hadith ends with the words:
And thus He continues till [the light of] dawn shines.
On the authority of Anas (may Allah be pleased with him), who said: I heard the Messenger of Allah (PBUH) say: Allah the Almighty said:
O son of Adam, so long as you call upon Me and ask of Me, I shall forgive you for what you have done, and I shall not mind. *O son of Adam, were your sins to reach the clouds of the sky and were you then to ask forgiveness of Me, I would forgive you. O son of Adam, were you to come to Me with sins nearly as great as the earth and were you then to face Me, ascribing no partner to Me, I would bring you forgiveness nearly as great at it. *
It was related by at-Tirmidhi (also by Ahmad ibn Hanbal). Its chain of authorities is sound.