Dear Mr. Jagadeesh,
The first step is to file the complaint and get the FIR registered which you have already done. Since the FIR is registered against unknown persons it is the duty of the police to investigate and trace out the illegal occupier and file the charge sheet. If the police is dragging their feet, you can move the High Court by way of apetition U/s.482 Cr.PC seeking direction to the police to complete the investigation and file the charge sheet. This will not take much time (maybe 2 to 3 weeks to get the order)
During the course of investigation you will get the details of the illegal occupant and then file a suit before a competent civil court for declaration of your title to the property and for recovery of possession. If you are lucky, the illegal occupant may agree to hand over possession fearing criminal prosecution on the direction of the High Court.
Thanks,
Ramakrishnan, ADV