If you want visible abs, you need actual muscle mass and < a body fat percentage under 10%. Just look at people starving in Africa—they aren't walking around with six-packs. Doing nothing but crunches is just a great way to mess up your pelvic tilt. Here's the breakdown:
In my opinion, you should be hitting full-body workouts. My recommendation? Push-ups and pull-ups. If you can't manage a single pull-up yet—don't sweat it—just use a chair to pull yourself up and then focus on a slow, controlled descent. Throw in some squats with a medicine ball and some lunges. For core work, try this:
and this:
Set aside an hour for your training.
Work every muscle group evenly. Aim for 5 sets of 15 to 20 reps. Keep your rest periods between 30 seconds and a minute.
There are plenty of variations for pull-ups and push-ups—you can easily integrate dumbbells or resistance bands into your routine. Honestly, Google and YouTube are your best friends here.
P.S. Squats engage your lower back, so there's no need to isolate that area separately.
Dedicate one hour and get it done. For cardio, just go for a brisk walk.