Please, someone help me!!! I don't know what to do, I think I'm in big trouble!!?

SCAM! ignore it. The FBI would never send out a letter like this. I work in a criminal defense firm as a paralegal for the last 12 years. We handle many, many federal cases and as such I have to work closely with the FBI and the United States Attorney. I can ure you they would never investigate someone in this manner. So relax and have a nice evening.