How to find out who sent an email - someone harring me?

Someone sent an email to my sons father saying nasty stuff about me and that if he wants custody of my son that now is the time. And saying all sorts of stuff that is not true!! Also about 1.5 months ago someone called dcf on me saying a bunch of stuff that was not true either. Well idk who it is, what do i do? someone is out to get me and try to ruin my life. My kids are my whole life. How do i find out who it is l, what am i supposed to do? im kinda scared