We as muslims believe that there is none who should be worshipped except Allaah (the One and Only God.) And we should do good deeds, like pray our 5 daily prayers, fast, pay money and help the poor etc.
Every thing we do in this life is witnessed by Allaah Almighty, and this is why we have to try to do as much good as we can to please Allaah, and try to stop ourselves and others from doing evil - otherwise Allaah Almighty may be displeased with us.
We believe that after every person has died, there will be the final gathering of all the people that have ever lived on the world - every person will be judged by Allaah to see what the person did in this life, was it good or bad? Did they believe in Allaah or did they reject their own Creator?
The ones who did evil and disbelieved in Allaah will be punished in the hellfire, and the believers who did good will be rewarded with paradise where they can have anything they desire.
We believe that since the beginning of time, Allaah has sent messengers to this world to warn the people of the day they will be judged, and told them that they should worship Allaah Alone, and not any other. But the people who disobeyed Allaah - they turned away and they will be judged. The same way the believers will be judged, and each person will be rewarded or punished justly for what they did in this world.
This is why we believe that this life is only temporary, so we should work hard to do as much good as we can to please Allaah. But the people who don't believe in Allaah, or the afterlife think that we only live once - but then you have to ask yourself, why was i created for in this world? And the answer to that is to please Allaah, if you do that - then Allaah will grant you paradise, but if you reject Allaah - then you will be punished for rejecting Allaah Almighty, even though the signs were shown clearly to people in this world.
This is why Allaah Almighty says in the Qu'ran that the believers:
Those who remember Allah standing and sitting and lying on their sides and reflect on the creation of the heavens and the earth: Our Lord! Thou hast not created this in vain! Glory be to Thee; save us then from the chastisement of the fire: (Qur'an 3:191)
The believers look at the world and know that Allaah did not create this world for nothing, but it was for a reason - and that reason is that we should worship Allaah,
And I (Allâh) created not the jinns and humans except they should worship Me (Alone). (Qur'an 51:56)
This worship is in many ways like praying, fasting, helping the needy, being kind to your brother etc. Anything which is good that will please Allaah.
If the people do good in this world and do it to please Allaah;
Allah will admit those who believe and work righteous deeds, to Gardens beneath which rivers flow: they shall be adorned therein with bracelets of gold and pearls; and their garments there will be of silk. (Qur'an 22:23)
But those who disbelieve don't believe on the day of judgement, so they feel they can do anything they want because they don't believe that they will be judged on what they do:
But they reject the hour, and We have prepared a burning fire for him who rejects the hour. (the day of judgement) (Qur'an 25:11)
Allaah Almighty knows best.
If you have anymore questions, please do ask.
Peace.