1. If the land is not a Govt. property, then certainly it belongs to some one. Find out from the Registrar in whose name title of the said piece of land stands,
2. Even if your father has taken care of the land for 15 years, it's title is not conveyed in his name without registration of a valid deed of conveyance,
3. Find out who is the real title holder and file a suit against him claiming adverse possession of the said piece of land and along with the said suit, file a petition under order 39 Rule 1 & 2 praying for an order restraining the actual owner and also the said miscreants to enter in to the said property till the disposal of the suit,
4. You shall have to establish your right title or interest on the said piece of land to justify its occupation by you and no body is bound to show you their/his title to you if you yourself do not have a valid title of the said piece of land.
5. So, first try to create a valid title on the said land by adverse possession claim.
6. Engage a local lawyer having expertise in this field to deal with the matter.
7. The cost of filing the suit and stay petition may cost you any thing in between Rs.40 K to Rs.80 K. depending on the quality of the lawyer you shall engage.