Like the other user said, sit ups work great! Those will mainly work the upper abdominals but for the lower abs, I recommend doing leg lifts where you lay flat on the ground as if you were doing a normal sit up, then just lift your legs up slowly, contracting your abs, and without using momentum from your legs try to push your feet to the ceiling and lift just your butt off the ground. It doesn't have to be very high, just enough to feel it in your lower stomach. Then slowly go back down. The most important thing to do is have good, controlled form. High reps won't matter if your not doing them the right way! Just keep at it! Good abs take time but if your just starting out you will see quick results if you do workouts daily!
start by doing about 10 sit ups every day. after about a week or so move up you sit ups to 30 or what every you feel get so dumb bells that are about 10 or 20 LB (pounds) or what you feel is good and hold one by your chest and do sit ups if you put your feet under a bed you get more leverage and you don't have to keep pulling you feet up all the time. i can do about 50 sit ups with a 20 pound dumb bell on my chest and i have been doing that for about a month and i have great ads and a good core.