If you have Murti or statue of Shri Kuber then it can be used for Puja. If you don't have Kuber Murti then as an alternate you can consider chest or box of Jewelry, representing Shri Kuber itself and worship it. Before you start worshipping the chest, you should draw the sign of Swastik with Sindur and tie Moli (Kalaya) to it.

The first ritual to be done before starting the construction, is a Vastu pooja or Kuber Pooja. This is ritualistic and should be done at an auspicious time and on an auspicious day, as fixed by your astrologer (normally it is done in the morning). The pooja should be done in the North-East corner of the site. The pooja should not be done if the woman in the household is more than seven months pregnant.