Depends on how long you've owned your apartment as Income Tax as it is now called is paid in arrears.
As non residents we have to pay 2 taxes, the first one wealth tax was set to 0% with effect from 1st Jan 2008 so last year we didn't have to pay it. It may be ressurrected at a later date so look out for any news reports.
The second used to be known as La Renta but is now called Income Tax. This is worked out on the Cadastral Value given on your Suma Bill (September).
How this calculated is explained in detail in the section further down under Living in Spain....Income Tax, there are also people listed on there who will fill in the form for you.
My solicitor used to complete this form for me but as the yearly amount I pay works out less than the amount charged, I too will be looking at either doing it for myself or using someone else.