I need help with my lawn!?

I know I have St. Augustine grass, and thats about it. There are big bare patches in the front yard and the grass in general is just weak and dying. I'd like to make my lawn much healthier and eliminate the big patches. I don't water it reguarly, which is probably a contributing factor. I was just wondering if there was anyone out there who knows what to do? Maybe sod? Mulch? Fertilizer? Seed? Are they all the same thing? I don't know, I just want to know what course of action to take here and learn about it in the process. Thanks so much.