The basics of getting abs
Abs come from low body fat and large core muscles. Ultimately weight doesn't matter, though checking your weight can help with achieving the low body fat.
To get abs, you need to first need to cut enough fat that your muscle tone becomes visible. This is best achieved through the combination of healthy, disciplined, and goal-focused eating (I don't like the term diet - it connotes a temporary habit) and cardio. Along with that you need to build up your core muscles. I stress build up, as if you don't have large core muscles, you will never have abs even if you have zero fat.
A tip for getting good health and fitness - don't bother chasing after "special" foods or exercises. You won't get six pack abs without sweating hard and eating healthy - if you do somehow, then you'll still have your poor cardiovascular health to sort out.
You should cut fat and build muscle at the same time as there's no point going from excess fat -> scrawny -> cut like a rock when you can go from excess fat -> cut like a rock.
Step 1: Cut body fat
To achieve this, the most important thing is to burn more calories than you eat. So of course this needs a two-pronged strategy of eating well and burning calories. You also need to make sure that your calories are consumed in healthy forms.
To eat well, first cut out all the bad stuff like fast food, sweets, and other calorie-intense meals (e.g. lasagna, big portions of trashy carbs like white rice, etc.). These foods make you feel worse anyway. Try be quite radical with this bit, none of these foods should be eaten regularly if you really want to lose fat. Now add in all the good stuff (lots of veg and fruit, good carbs like oats and wholegrain rice).
After you've done that, you need to watch your calories. Try keep track of them in a simple manner and do a bit of read up on calorific content of different foods. You could be quite surprised. Also get used to reading the nutritional information on packets. I'm not a fan of fully detailed calorie tracking, as I don't think it's sustainable unless you're OCD.
For the cardio bit, you want to be doing some good cardio exercise every second day for a substantial period of time. Aim for an hour at least. Find out what your thing is, whether it be running, swimming, or cycling. If you find exercising on your own difficult, maybe do a team sport or get a group of people to exercise together with. Make sure it's a group of people who are keen otherwise they'll just drag you down.
This step is challenging, as eating calories is surprisingly easy and burning calories is surprisingly hard. Have a look at some calorie websites to get an idea, and get some people or even professionals to support you in this.
Step 2: Build muscle
This should be done through exercises that get all your body, and some core-focused exercises. It's not good to just build a single muscle group.
Start by getting good at an endurance level of repetitions (20+), then move to more strength and muscle growth repetitions (10 very hard repetitions). A common error is to do huge numbers of reps - this will do little to build muscle size.
Basic sit ups are are a great start, do three sets every second day. Here's some more ab-focused workouts:
- Twisted sit ups - twist your torso 90 degrees one way as you reach the top. These are great for your obliques.
- Lifted leg sit ups - lift your legs up so that your shins are floating parallel to the floor, and thigh is perpendicular to the floor. You can get quite creative/masochistic with this one - stretch the legs out horizontally but not touching the ground, then all the way vertically, etc. It's fun!
- Medicine ball sit ups - hold it above/away from your head, sit up and throw it against a wall and catch it
- Big V sit ups - whole body is straight, then your body goes up like a V pivoting at your waist
- Floating flutter kick - lying on your back, legs out horizontally but not touching the ground, kick your legs like you're swimming with knees straight. Do this for 3 sets, as long as possible
- Squats - start without weights, then add weights once your style is good. Great for core strength and stability. See here for a great guide to proper technique.
- Gym machines - there are a handful of core-burning gym machines, they usually focus on a sit up movement but add weights. These can be very effective.
First get your reps up to maybe 50+ in a set for easy sit ups, then start doing more challenging ones, like the big V, where you can only do 10 reps. The medicine ball is great, as you can just get a bigger medicine ball when it's too easy!
Diet is very important at this stage. Once you've reached a low body fat, your calories eaten should equal your calories burnt. You now need to make sure you have a good intake of protein. Your mindset needs to move from slimming to bulking up with muscle.
Swimming is brilliant for abs
All those sit up exercises came from my years on the swim team in high school. We took it quite far, including some sets of 1000 sit ups. This isn't necessary for nice abs! Back in those days I had a good set of abs, including the Vs of the obliques. My abs are still evident now, and I don't do heaps of exercise - I just have avoided getting fat, and make sure to fit in some exercise when I can. I'm blessed with a high metabolism, but with correct eating and exercise anyone can achieve this.
Swimmers need rock solid core muscles, as that's what enables our legs to move relative to our torso. Here's a few places where core muscles are vital:
Dolphin kicks off the wall for most strokes. These are powerful, double-legged thrust-like kicks underwater. Your core muscles basically need to move your legs back and forth as hard as they can and as quick as they can.
Dolphin kicks while doing fly
Controlling rotation for freestyle and back stroke. With both these strokes, you rotate your torso as much as 90 degrees to enable you to get the maximum pull. Your core needs to control this rotation.
Along with core-focused muscle work, swimming burns a good deal of calories, and also works all the other major muscle groups in the body, particularly shoulders. It's also a great sport because of the lack of physical impact.
Don't forget the rest of your muscles!
If you only do sit ups, you'll end up with a fairly hunched over physique. Make sure you work your back just as much, and while you're doing that, just work the rest of your body. May as well get your whole body in good shape!
Here's a few approaches, from best to worst (and probably least accessible to most accessible):
DEXA scan - this is a type of X-ray, so you need to go to some form of medical practitioner for this
Underwater weighing - this works off body density (fat floats), though this will make someone with dense bones and muscle appear to have less body fat than they really do.
Skin callipers - these are big pincer things that measure the thickness of your skin. With measurements taken in certain areas (at least 5), along with height and weight, you can calculate your body fat using a guide. There's an art to measuring the skin correctly, so it may be best to go to a professional.
Electronic scales - these often have a nifty way of measuring your body fat, probably by sending electrical signals through your body.
I never really got into measuring body fat, I've only had it measured 2-3 times my whole life. Usually you can just look in the mirror and tell how you're going.
I also found the calculation to be unhelpful. I was measured at roughly 11%, but I had virtually no fat. All the calculations are indirect approximations, correctly calculating might require you to be dead!
Though as pointed out by Moses/JohnP, they are usually off by a fixed amount, so you'll still be able to track your progress to some degree.