Old Septic Tanks & Fields
First call is to the local building department, to find out what is allowed in your area. Some places let you fill them, others make you remove them.

If you have to remove it, you'll have to check if it's empty. If not, you'll have to call somebody out to pump it. Once empty, you'll have to call a bunch of people with shovels, or somebody with a really big shovel.  You'll also need somebody with a crane, and a big truck to haul it away.  You could ask at the building department for references for local companies that might offer removal service.

If you can fill it, follow the filling instructions of the local building department