Are you asking if you need 'equipment' to acheive it? Or are you actually wanting to do ONLY situps?
You don't need equipment, persay, but you do need more than situps alone.
You should do at least 15-20 minutes of medium to high intensity cardio. Whether running, some form of tae bo, something to get your fuel going, THEN, you'd want to do sit ups, planks (look up plank exercise on YouTube if necessary), Arm to leg cross overs, push ups, sit ups and obliques, then, if possible...10-15 more of cardio. It really makes the difference.
Do reps of whatever you can. It should hurt afterward, that's good. It means you're pushing your muscles to do real work. Increase your reps as your muscles get used to it or you won't be building anything new and only maintaining.
Youtube is great to find out how to PROPERLY execute the excercises so that you're getting the most out of them and not staying in your comfort zone. You shouldn't be straining your neck or back, or legs, but using your abs and core to pull yourself up.
Practice makes perfect! Good luck. (: