Since you are not used to working out, you should take it easy and build up stamina gradually, as opposed to pushing yourself to the limit, right from the start. The key to staying fit is to have a **pleasant** routine you enjoy. (Find a new routine when the first one gets boring.) Since you are sore at the moment, do sit-ups every two days, and not too many of them. You don't want to be sore two days after. Also, the traditional sit-ups only train the top segment of the abs. If you want a full ab workout, your series needs to include a variety of movements. On the days where you are not doing sit-ups, exercise other muscle groups. There are many interesting floor exercises explained on YouTube.
yes, that means the workouts are working. dont over do it tho. know your limits
Start off with doing 50 then after a week or so increase by another 25 then after another week add another 25 and so on and build it up slowly . It sounds like you are trying to run before you can walk.do it gently and you shouldn`t hurt so much. Spread the exercises over alternate days-say mon wed,fri then rest over the weekend and start again the next week otherwise you will burn yourself out, be patient , everything takes time.If you damage your body then it can take time to heal and will set you back. Everything in moderation.