You can't sue unless you have filed your copyright notice with the LOC, but it's not necessary to have filed immediately upon creation in order to sue.
Lawsuits are difficult to win.
You can not copyright an idea, a word, or a title. Copyright violation is almost always a civil matter, so you have to spend big bucks on an attorney to sue, and it is unlikely you will win. In general, copyrights now last for the life of the author, plus 50 years. The old wives' tale of mailing the manuscript to yourself doesn't hold up. You might at least have some proof if you got the postal clerk to handstamp the postmark all along the sealed edge, but don't rely on this as proof in a court of law.