I had a roommate that signed an agreement to pay their half of the rent and utilities. They did not pay. How do I go about a lawsuit?

The statute of limitations is 5 years from the date they breached the agreement, if it was a written agreement.  Of course, you have to prove it was a written agreement.  File a Warrant in Debt in District Court-the worst that happens is you lose your filing fee. I'd suggest writing them first telling them you are going to file if they don't pay and perhaps that will stir some action.