Seperation papers before I leave or after?


Do I need to have seperation papers before I leave with my 3 yr old son or after? We have discussed me moving out to live in my parents extra house but I need to know in order to do it right should we agree on things before I leave or is it ok to go ahead and leave and then settle on agreement? we both want joint which is good considering it would be a great help, but my question also is if I move out do I still have to help pay for the household even though I am not living there (the house is in both our names)?

Any response would be greatly appreciated.