Oh my goodness.  Please, please, please go and get an attorney here.  I hate to sound as if I am police bashing and not trusting of our law enforcement here but I really think that you need to have someone in your corner solidifying the deal and making sure that you will not in fact be charged in exchange for your help as an informant.  A deal IN WRITING and signed by the party or parties that will do the charging:  the District Attorney or County Prosecutor's office.  The police can offer you a deal but they are not the ones that will charge and prosecute you.  So please: get help.