While doing sit ups/crunches and I don't know what they are called but you start in the sitting up position and then go half way down and back up again. <------- and if that didn't clue you in to the fact that I'm a workout novice my next question might.
What's a decline bench?
Well, it is a variation of sit-up. I would recommend that you start from the lying position and raise up halfway, then lie down again. That is a crunch, and puts more tension on your abdomen.
The decline bench looks like this -
(With the barbell, for the decline benchpress)
(Without the barbell)
This is better for crunches, as you can set the slope using the yellow knob at the foot of the bench. Just pull it, let the bench drop to a slope that is comfortable for you, and click the knob into a slot. Remember - 1. the steeper the decline, the tougher it will be and the greater the intensity of the workout, 2. Lock your feet in the sponge-pads provided and keep your feet tensed as you go crunch/sit-up.
If bodyweight crunches or sit-ups are easy for you (15 reps per set), then hold a plate to your body to add weight. You can start with a 10lb plate, and then work your way up to 25lb, 35lb and 45lb. Ideally, a set should should have between 12-15 clean reps without any assist.
You should find a decline bench in your gym. It is usually in the part of the gym where the big boys work out. If you do not want to go there or if you gym does not have a decline bench, you can use an exercise ball to do the same workout. Here is a gif (not sure if the pic is appropriate for CC!) -