Separation agreement and different counties


My ex and I are signing our separation agreement today and getting it notarized. He lives in a neighboring county. Where do I file these papers? When I start the divorce proceeding…where do I file those papers?
Also, I learned on this website that if he lives in a different county that I have to get the Sheriff to serve those papers to him. I guess that means the Sheriff of the county HE lives in? I just want to make sure I do all this correctly so I can get this divorce ASAP.
Thanks for any info!