How to file a civil suit for neglect of repairs against my landlord?

Hello everyone..Can someone please help me, preferably an attorney or someone with legal experience. I am in Houston, TX and the central a/c unit in the property I rent has been out for 3 weeks. My Texas Real Estate Commission Lease agreement clearly states that all a/c repairs are to be completed by the owner unless there is negligence on the behalf of tenant, i.e neglecting to change filters, which i have done every month. I am not behind on my rent and my landlord keeps giving me the run around. I would like to file a civil suit to make him fix it, or pay me damages if I have to move. As you know it is extremely hot here in Houston, and I live in a town home and window units are out of the question, because I hardly have any windows upstairs and none downstairs. I just want to know what steps do I need to do to file suit. I know I need to send them a certified letter first, but what should it state. Thank you so kindly in advance.