Sit-up and crunch are the most common abs exercise, the following link show you how to do it correct way.
http://www.youtube.com/watch?v=bbFJRIZ4qD0&feature=related
http://www.youtube.com/watch?v=TKg_cdwq9l4&feature=related
There are many equipments you can buy, but I think the exercise is a very effective way to have the abs you wanted.
Keep in mind that abs exercise alone is not good, many trainers suggested people should train the whole body, not focus on one part, also you should add cardio exercise on your routine, then you can lose your body fat and show of your 6 pack muscle.