If I use a free copy right site to copyright my material, is it still considered illegal if someone tries to infringe my original works?

The registration of a copyright is not required for it to be enforceable.  The "copyright" is created at the time the work is created, irrespective of whether you use a "copyright site."  However, the question of whether you have any rights against the person that is infringing your work is very fact dependent.  In other words, we can't tell from your question whether you have a claim that would prevail in court.