Possible Identity Theft/Forgery

I purchased a motorcycle in my name and a friends name a few months ago. The other night i recieved a call from the finance company stating i owed 450.00. I know that i have paid all the payments on time over the phone, so i checked my banking information to make sure all the payments went through, which they had. I called them back to try and understand where they were coming up with that figure. They stated the payments went up due to two purchases after the bike purchase. We have not purchased anything on that account, so i am disputing it. The rep at the finance company pulled up the sales reciepts for all purchases and stated the other purchases (the ones im disputing) have both of our signatures. She looked into it further and compaired signatures; she said the purchase was for an extended warranty (2 warranties back to back). We are assuming the sales rep who sold us the bike gets a huge incentive for selling extended warranties, and cut our signatures off and pasted them on the extended warranty sale paperwork. The reason we are thinking this is because; number one, we never signed it ourselves or agreed to it; number two, the date with the signature is the same as the date on the extended warranty. The rep with the financing company also said the lines on the page look like they were put in themselves. We are waiting for copies of the bill of sales for all purchases to come to us and then they stated we need to contact the fraud department when we receive them. I was told by a friend of a friend that i need to talk to the magistrate and file criminal charges against them. Can someone help with more information or what i need to do to get this taken care of?? Thanks in advance!