All religions for the most part ask you to do good on to others and contribute to the world in a positive manner.
Islam does that but also requires you to believe in One God, he alone we worship and his last messenger is Prophet Muhammed (peace be upon him) whose example is the best of all men.
We do are not accountable to anyone but the One God (Allah in Arabic). There is no one who can forgive our sins but him. This life is temporary and in it we must do good deeds as well as fulfill our duty of worship to God and then there will be a Day of Judgement. There God will make the final decision about our afterlife—whether a person will go to heaven or hell.
Hope this helps a little bit. If you have any specific questions, please feel free to ask.
This applies to all those who die and do not repent for their shirk because this life is a test and it is during this time period that we will try to refrain from committing sins, but if we do, then we must also use this time to repent before it is up.
Allah (SWT) has told us that he will forgive all sins for the one who repents to Him. He says:
“Say: ‘O ‘Ibaadi (My slaves) who have transgressed against themselves (by committing evil deeds and sins)! Despair not of the Mercy of Allaah, verily, Allaah forgives all sins. Truly, He is Oft‑Forgiving, Most Merciful’” [al-Zumar 39:53]
This includes all sins, even shirk. Whoever repents, Allah will accept his repentance.
Hope this helps :) May Allah make it easy for you, Ameen.