According to the Hindu religious tradition, the northeast corner of the house is considered to be largely associated with bringing happiness and prosperity for the family members. Therefore do select the northeast corner of your pooja room for worshiping the Ganesh idol. This corner is also considered to be much calm and clean.
In order to attain the sense of purity at the spot where the Ganesh idol is placed, a day before the festivity cleans that area with pure water or Ganga water. Do sprinkle sufficient water in that area and manually clean the floor and walls.
You can set a colorful background by full covering the walls with yellow, red, green, white or pal wall papers that is readily available in the open market. The green and yellow theme is the right color of Ganesha to decorate puja ghar. It is to be noted that green is the best color for Ganesh Chaturthi.
For floor decoration, you can place a colorful piece of cloth or red carpet on the floor. You can also place a brass or wooden shrine on the ground.
You can make the shrine look bright by placing diyas (oil lamps) to all the corners of the shrine in the pooja room. In order to get more brightness and shine, do cover the shrine area with red or pink colored velvet cloth.
Finally place the Ganesh idol on the center of the shire and light a lamp. You can also add natural flowers and garland to the Ganesh idol.