It will be decided on the day of Judgement that who will go to Jannah and who will go to Hell By Allah SWT. Holy Quran and Hadiths of Or Prophet SAW describes the detail of Jannah in detail.
According to Hadith there are eight different grades of Jannah in which Jannat-ul-Firdous is the highest while Jannatul Mava is the lowest. People will enter in different Jannahs according to their deeds. The Jannah is made of Bricks of Gold and Silver. There will be big beautiful gardens and houses in Jannah. There will also be Canals of Milk, Honey, Water and Sharabun Tahoora. There will be no diseases or death in the Jannah and people will always remain happy. There will be tasty Fruits and other foods in Jannah.
Every Human will remain young In Jainnah. People In Jannah Will be Beautiful, Sweet and Long in the Lengths. The People in Jannah will wear Beautiful cloths. There will also be no Urine or stool. There will be Hoors in the Jannah and every Hoor will be far more beautiful than any women of this World. This Information About Jannah is taken from Hadiths and Quran. For More Detailed Information About Jannah you can rood books on this topic. May Allah SWT send us to Jannat-ul-Firdous in hereafter.