Should I pay GST on purchase of commercial property?

I bought a coomercial property at the end of January 2008. I have been collecting rent on it since then and I collect 5% GST on the net rent that I charge. I was going to fill out the GST form which is due at end of March but I noticed in my purchase doents that no GST was paid at the time of purchase. Do I pay 5% GST now, less the GST that I have collected during 2008? Do I keep deducting the GST that I collect for rent in the next years from the large sum I should presumably pay now during the following years?