I made a lot of mistake in my life, but now I m trying to be a good Muslim. But my problem is this: I am not afraid of god, I know it is wrong. I don't know why I should try to be good?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Fearing Allah is one of the most precious results of faith in Allah. Belief in Allah produces the fear of Allah, observing the limits set by Him. The more a person increases his knowledge about Allah, the more he fears Allah. Allah has ordered His slaves to fear Him. As He says: "so fear them not, but fear Me, if you are (true) believers. (Aali Imran 3:175)

In this verse Allah made fearing Him a condition of the true faith. Allah says: "Therefore fear not men but fear Me." (Al-Ma'idah 5:44)

Allah eulogizes His nearest slaves of fearing Him, as He says: "Verily, they used to hasten on to do good deeds, and they used to call on Us with hope and fear, and used to humble themselves before Us." (Al-Anbiya 21:90)

Allah also describes angels that they fear Allah, as He says:  "They fear their Lord above them, and they do what they are commanded. "(An-Nahl 16:50)

There are many verses and Ahadith that encourage one to fear Allah, because the more a slave knows Allah, the more he fears Him.

 Imam Ibn Al-Qayyim wrote: "The decrease in the fear of Allah is due to the decrease in cognition of Allah. The best one knows Allah, the most he fears Allah, loves Him and respects His rulings."

Firmly believing in the Hereafter also leads to God fearing. The one, who believes that he will surely stand in front of Allah, believes that the Paradise is prepared for pious people and Hell fire is arranged for Kuffar, and have faith that due to sins he will go to Hell fire, who believes so will fear Allah (undoubtedly).

We do not think that the questioner brother is not afraid of this world fire, how will he not be afraid of the Hell fire if he truly believes in that? The above details prove that not fearing Allah springs from weakness in faith and not having proper knowledge about Allah, and His attributes.

For more details please read the fatawa: 84101 and 85477.

Allah knows best.