First to answer your questions.
A. Depends entirely on the situation/person I'm talking too. I think its about striking a balance between being rude/considerate of the other persons feelings and plain honesty.
B. How big is "big"? I like curvy gals, but I'm not exactly turned on by morbidly obese people :/
C. Getting the mental motivation to actually do something that will make a change to your lifestyle is hard! It can be even more stressful if you expect too much too soon. (You're not going to loose 3lbs overnight). Also if you don't keep up your routine once you lost the weight you could just end up putting it right back on. You need to bear this stuff in mind before embarking on a weight loss journey to avoid getting bummed out.
Second
If you want an honest opinion from someone on the forum about the way you look, why not post a photo with any determining characteristics blocked out. e.g. face (Ms Paint o_0 ?)
Finally
Sit-ups may help develop abdominal strength but they won't "cut" the fat around your belly. Weight loss just doesn't work that way. Without a change to your diet and a regular workout routine/regimen, I doubt doing Sit-ups by themselves would get you the results you are looking for and could just demoralize you.
(also what is more boring then doing endless sit-ups.. erugh)
If you go in with a half-assed attitude you’ll get half assed results. (or even quarter assed :O )
For a good set of results you should look at designing a plan which incorporates both cardio and resistance training. No, lifting weights will not turn you into a super butch lady built of muscles; forget about it, you don't have the testosterone for that to be a possibility.
It will however complement the cardio and aide your weight loss.
Once you have your routine stick to it! 90% of people give up after about 3-4 weeks (yes I totally just made that statistic up, however I do get the impression that’s how long most people put into it before giving up.)
No a gym membership is not essential for weight loss. It can help, but it can also be a hindrance (there is a strong tendency to go to the gym with friends and spend most of your time chatting/mucking about when you should be pushing yourself as hard as you can.)
If you’re stuck for ideas about how to exercise without going to the gym google is only a few clicks away
Dang Scarlett beat me to the punch