You have learnt already that we should believe in One Allaah, in Rasulullaah (peace be upon him), in Jannah (Paradise), Jahannam (Hellfire) and the Aakhirah.
Without believing in all of this, a person will not be a Muslim. You already know that good people will be rewarded by going to Jannah and the evil ones will be punished in Jahannam.
On the other hand, Jahannam is for those people who do not believe in Allaah and who do evil. Although such people also get punished in this world, Allaah has prepared a severe one for them in the Aakhirah.
Rasulullaah (peace be upon him) told us that the fire of Jahannam is seventy times mores severe than the fire of this world. Mishkaatul Masaabeeh (Pg.502).
Does anyone like to burn himself in the fire of this world? When some people burn to ashes with the fire of this world, imagine what will happen with the fire of Jahannam!
During summer we see that people do everything they can to keep the heat away. They look for shade, use fans and air conditioners because they cannot bear the heat. The heat of Jahannam is so severe that even the sun gets its heat from it.
Rasulullaah (peace be upon him) said that the person who will suffer the least punishment in Jahannam will be someone who will be given sandals of fire to wear, the straps of which will also be made of fire. Because of this, his brains will boil like how a pot boils on a stove.
Although his punishment will be the least of all the people, he will think that none is suffering as much as him. Mishkaatul Masaabeeh (Pg.502).
Jahannam is a terrible place that we must all beg Allaah to save us from it. We must recite the following du'aa:
"O Allaah! I seek your protection from the punishment of the grave and the punishment of Jahannam."
A Hadith tells us that Allaah allows Jahannam to hear these du'aas of the people and Jahannam also makes du'aa saying,
"O Allaah! Save him (the person making this du'aa) from the Fire."