We need more information.

What kind of material, to start?

Checking the various places where current works appear will give you copyright information for a lot of things. Pick up a book and see who copyrighted it and/or check with the publisher. Same with musical recordings, etc.

Are you trying to find if something old is out of copyright? Then be a little more discrete if you're trying to get copyright for yourself, because if it is truly out of CR then is could be up for grabs. Alerting a publisher in interest in something they've allowed to lapse may have them hastening to renew.

There are attorneys that take care of this for big publishing houses. A friend of mine worked for a company in New York City called Kendal and Kendal that at the time handled Marvel Comics. They nearly lost a huge character because no one noticed copyright was about to expire. It was a last ditch effort to get it filed in time. This was back in the early-1980s.

SRS