Abs are one of the most noticed body parts. Especially by the gals. The love a nice strong six pack to grabs and rub. The problem is getting them showing.
Well the truth is that we all have great abs. They are just hiding away under all that skin. You need to have quite a low body fat percentage and the determination and dedication all bodybuilders need.
I have read in the message boards that some teens do maybe 50 or 100 sit-ups in 1 go. This makes injury high risk. A fiber could tear in the muscle which will caused tremendous pain. I suggest doing 4 sets of 25. Go up slowly and breathe out and come down slowly and breathe in. Also when you are at the top of the sit-up hold it for a second. If you do this you will feel you abs tighten after every set. I feel crunches are the best exercise for your abs following the same rule.
Including twists in a sit-up is a good idea as this will work the external obliques. I suggest only doing a twist in a sit-up and not a crunch.
I work my abs every workout I do which is 4 a week. For a beginner train your abs twice a week with good form.
Keep at it and never give up. Make small changes which will eventually add up to a big complete physique change.
Keep up the good work.