Before you start the process, check with the title company and a surveyor to make sure that this is really your property. I wouldn't just take the word of a neighbor. You will ultimately need to get advice and representation by a real estate attorney. Best of success.
Robert McGuire ASR
Your Castle Real Estate
Direct - 303-669-1246
You won't get anywhere without a lawyer - nobody will take you seriously.
I have been in a similar situation and used Darrell Damschen as my attorney. His website is http://www.madlawfirm.com
I had to file a lawsuit over an issue that was similar to this and used Darrell after interviewing five lawyers. He is easy to work with and fair with billing. He'll tell you right up front if he can help you or not.