WA state residential landlord tenant law, tenant's rights against forged lease by landlord

My former landlord screwed up and didn't have my girlfriend and I sign a lease for our apartment. We asked multiple times and it never got done. We were sick of constant rent raises, maintenance taking a long time or not at all and decided to move.
After we moved into our new place we recieved a letter and hoped it was our deposit, but it was a move out report saying we didn't give a 20 day notice and returned the keys 4 days late. We were charged for the next month's rent and the document went to collections.
The collections agency informed me that I had signed a lease and she was looking at it right then. She made copies and sent them to me. Everything is forged. My initials down the side and signature at the bottom are blatantly fake and my girlfriend isn't on the document at all. I have a carbon copy of the move out inspection where both my girlfriend and I are listed. On the collections copy, her name is scratched out. I also have a copy of the disclosure notice where both of our names are written and my actual signature is listed. This shows the amount of money I paid for the deposit. This document was not in the forms the collection agency recieved from the apartment manager.
I filed a police report for fraud and don't know what to do next. What type of lawyer do I need? Do I sue for the money owed to me from my deposit since it's the only official signed document? Since there's no official form showing the amount of time I need to give when I move out and nothing saying I'm to return the keys the night I move when he's changing the locks anyway amI liable for these charges? My credit rating is hurting now and my rental history is shot until I clear this up. Where do I go from here?