You sound sightly underweight
so increase your food intake but not the sautrated fats because that will be stored. maybe proteing to help with the muscles.
For a flat stomach
do crunches until the "burn" point and then do 10 more. burning being the pain you start to feel. The extra 10 will be challenging you more and therefore better.
do reverse crunches, pulling your knees to our chest instead!
Do twisting crunches, same as normal except you take your left elow to your right knee.
Do "the plank", rest on your toes and forearms, hold your back straigjt (so your tummy is off the ground) and then old it baically, if you do it right you should feel it....you're tummy will shake a bit - it's awesome abs toner.
Running does the abs aswell.