I started with 100 situps and push-ups over an hour while watching tv and cutting dessert. One month later I looked in the mirror and was inspired to do more. Little things add up. Try something small, daily for 2 weeks.
100 situps is a lot for a beginner. Actually, after 3 years in gym I did ~80 in 4 sets. Not because I can't do more but 1) it is a high stress for a spine and 2) there are other exercises to do.
Sit ups can be really hard on the spine, even if done "properly". You slip a disc in your neck even slightly while working on your abs, and you won't be working on your abs or anything else for a long time. Slipped discs can cause an astonishing amount of pain.
Not an exercise guru, just someone who slipped a disc.