custody of my child

i live with my girlfriend we have a 6 year old.we are getting seperated and i want full costedy.this weeken she told me that she was going to idaho with her brother and she took my child.next morning i got a call from school.that my dauter was crying and need it to pick her up..i knew that shes in idaho. i went any ways and she was in school.i called my girlfriend and ask her were she was and who did she left my child with..and she said with a friend and she went to las vegas..she left the state with out telling me nothing.and we were living in the same house..please let me know what can i do to get costody/..