50 situps every day that wouldent do anything rite?

you should try different exercises. try doing crunches instead of sit ups. you will feel it.

Do many different types of excersizes: situps, crunches, pushups, pull-ups etc. and try to do more and more sets and reps each time

Well if you're the average person it would of course strengthen ur abs, but the answer above mine is right, u need much more variety, for a number of reasons. Firstly, it is very hard to stay motivated to work out, & doing the same thing every single day will become stale after a couple of weeks, so mix it up. Secondly, if you're doing the same sit ups for 50 reps, only one area of ur abs r getting worked, u need to exercise the middle, upper, lower, oblique, & pelvic muscles of ur stomach if you're looking to really burn calories and/or get a six pack. You do not even have to do this every day, 3 or 4 times a week of mixed ab routines is by far more beneficial than 50 ordinary situps 7 days a week.