Make sure you have a day or two off. And don't be too hard on yourself if for certain cercumstances you miss a day or two occasionally.
Keep a little notebook, this will help you keep a routine and check your progress.
And have a week or two completely off from your routine, every several months. Your body needs a holiday too. You'll find that you will be craving to get back into it after the break.
As for sit ups.
Try to place your feet up on the bed, so there is a 90 degree angle made at your knees. If your bed is too high (longer than your thighs) find a stool that you can use, or sofa(couch, setee).
Fold your arms across your chest so your hands are on the opposite shoulders (keep those hands away from that head).
Do NOT bend up. Lay straight as a board, then pull up with your stomach muscles. Keep your face/eyes looking up, not at your legs or groin.
The reason for this is to not bend your neck, or back. You are focusing solely on your stomach doing the work.
You won't see much movement, it might only be a few centimetres, but you will feel the strain, at first it will be very tough. You might only do a few. And exhale when you compress, breath in as you come back towards the resting position.
You can also go slightly to each side. Centre, left, centre, right, centre, left, repeat.
Also for stomach muscles it is the amount you do not how fast or the strain you put on. They are different type of muscles or something.