How does someone petition to get a county lined moved to expand another county? I believe it would be in best interest of our town, which has 2 counties running through it. Any ideas how to start?

The exact procedure varies somewhat from state to state, but it usually involves the approval of both of the involved counties, either through the county government or by a direct vote of the population of each county (sometimes both). There is probably an office of the county government (often called the county clerk or similar) whose job includes answering procedural questions like this; I would start there.

But expect this to be a major undertaking, and a political process at that. If you don’t have the wherewithal to collect petition signatures and that sort of thing, you may find it frustrating.