My primary motivation is results. I know that's kind of working backwards (you have to have motivation to work out, and working out provides results, the results motivate you to work out) but once you start to see results, that can fuel you BIG time.
As far as the abs, I'm a firm believer that diet is the primary factor in ab definition (and obviously I can speak from experience) so yes, a clean diet is essential. Nothing like junk food and beer to get that big belly you never wanted, right? In the gym focus more on compound lifts - kill the squats and deadlifts, IMO...anything that will work the core while getting a full body workout. Don't worry about crunches and whatnot, not yet anyway.
"Somewhere, someone isn't impressed by your looks. Not all men jump through the hoops of your fire. You're unbelievably boring to more people than you'll ever know." --Henry Rollins
"If someone tells you something is easy and they can't explain how to do it, it's not easy." --Steve Shaw