Why do people keep asking these stupid fucking questions? Just get on the goddamn treadmill and move around. It's not rocket science. The goal is to burn calories. You'll burn 'em walking on a flat surface and you'll burn 'em walking up a goddamn hill. Depending on how out of shape you are, walking on a fat surface won't get your heartrate up (e.g., not burning enough cals) until you're practically jogging. Incline makes it so you don't have to walk as fast to still get your heart rate up.
Don't overthink this shit. Want to lose weight? cut your calories and start moving around. "How" you move around isn't all that important.