That app sounds like a good idea. It'll keep you on a routine. Good old-fashioned push ups worked for me in the past. I started by doing them against a wall because I wasn't strong enough to do them on the ground. Then after a couple of weeks I graduated to "girl push-ups", then regular ones (which were still super hard, so I mostly the girly ones). You can switch up th position of your arms to target different areas.
My stomach got smaller, but I wouldn't say it got flatter. I've lost about 100lbs and my belly area has loose skin and some fat. I have about 50 more pounds to go. Unfortunately I don't have a good view of my private since the lower belly now kinda droops over that area. I haven't been doing any an exercises so that may have made a difference.