Everyone has sinned and we sin all the time so will we be punished for these sins? Or will we enter heaven without a punishment if we repent out sins?
Allah say in Quran:
On the Day We will say to Hell, "Have you been filled?" and it will say, "Are there some more,"(Surat Qāf,verse 30)
... "I will surely fill Hell with jinn and people all together.'"(Surat As-Sajdah,verse 13)
This verses emphasize that the promise of Allah will happen, and Allah will fill Hell with jinn and people; and of course they will be punished there. This punishment is for jinns and people who err from the right way without repent out sins.
And what will happen for people who repent out sins? If it be a real repentance, with regret and leave the sin forever; then Allah is tawab and will forgive him/her; and He/she can go to heaven without punishment, as Quran say:
And those who do not invoke with Allah another deity or kill the soul which Allah has forbidden [to be killed], except by right, and do not commit unlawful sexual intercourse. And whoever should do that will meet a penalty.
Multiplied for him is the punishment on the Day of Resurrection, and he will abide therein humiliated -
Except for those who repent, believe and do righteous work. For them Allah will replace their evil deeds with good. And ever is Allah Forgiving and Merciful.(Surat Al-Furqān,verses 68,69,70)
Imam Ja'far al-Sadiq (a.s.) said:"some times some one does a sin and Allah will send him/her to heaven because of that sin."
Means that person will become fear about that sin and does real repentance which Allah will accept it and won't punish him/her.
But pay attention, It isn't for those who sin all of their life and want to repent out sins near their death.
"But repentance is not [accepted] of those who [continue to] do evil deeds up until, when death comes to one of them, he says, "Indeed, I have repented now," or of those who die while they are disbelievers. For them We have prepared a painful punishment."(Surat An-Nisā',verse 18)
Allah accepts repentance from any Muslim so long as his soul has not left his body. I.e, as long as he is still alive. That being said however, Allah will stop accepting repentance from any Muslim once the sun rises from the west and sets in the east. This is one of the major sins of al akhira (the day of judgement), in which every Muslim's book will be closed, and his dua'a will no longer be accepted by Allah.
However, only Allah can forgive anyone's sins, and only He knows whether your sins can be forgiven. There is not any sin that Allah cannot forgive except for shirk (associating anything with Him). Keep in mind that before ibless got kicked out heaven, he asked a favor from God. He asked Him to keep him alive until the day of judgement to guide the human race astray, and God accepted his favor. Therefore, God accepted the favor of the worst of Allah's creatures, and therefore can accept any dua'a or favors of any of his human creatures.
Just make sure that when you make repent or make a dua'a that you are sincere to Allah, and that you will try your hardest not to repeat the same sin again. Surely Allah knows best, and He loves to forgive your sins. Allah is the All Mighty and Kind.
Please inform me if I've made a mistake. Allah knows my intention and only He can forgive me for anything that I may be in fault.