I'd start with planches if I were you...
Just look them up, haha. But yes, pushups are regarded as one of the best ways to work your body...using your own body weight is often the most effective - granted at a certain point you will want to push your body and increase the weight.
As far as showing abs, crunches will do nothing but strengthen and tighten the muscle. You could do 400,000 crunches and never see a single can in your six-pack. It must be accompanied by aerobic exercise and healthy eating habits (I don't like the word diet). If you can stay away from lots of cheese, processed foods, and sodas, you will see that you'll start losing lots of fat. Unfortunately there's really no way to isolate a certain area (e.g. your "tire" stomach area) but if someone can disprove me on this, please do! I WANT TO KNOW!
I've been going to the gym with two friends having never really gone before. I was pretty athletic before but never really "buff" per se and I'm already starting to see muscle tone in my arms and abs. We lift and do lots of abs/pushups and then go on to cardio (they bike, I run). When doing cardio, the key is getting your heart rate UP. Depending on your age, your 'target heart rate' could be anywhere between 130 and 220. I usually hover around 170-185. When you're bangin' on all cylinders at your target heart rate, you're a fat-burning machine.
If you do choose to run, intervals are the best thing to do (at least that I've found). Walk for a minute, then sprint for a minute. Do this for as long as you can or set a distance you want to reach; I usually aim for 1.5 miles and I've been increasing this. A mile and a half doesn't seem like much, but when half of it is sprints, you'll feel it. Haha.
Anyway, that's all for now.
