Hi and welcome!
It's great that you decided to lose weight, its so satifsfying

Well, your brother.. talks nonsense. Lol. It's not 500, or 1000, or 5000 situps a day thats going to get you into shape and become 'skinny'. Does your brother do serious
gym or any type of training? If yes then i get the impression he told you this to have a laugh because anyone fairly initiated in the world of
fitness knows that what he told you is just not true. The way you will loose weight and keep it off is to eat right (cut the friend foods, eat fruit and vegetables for snacks instead of candy and crisps and patisseries) and do excersize.
I dont know what you class as 'skinny', can you tell us what weight you would be happy with? You need to be doing some over all ground training like running or
treadmill if you are plannnig on going to a
gym/excersize bike to lose overall weight. Targeting a specific area like 'if i do a thousand sit ups every day in a month i'll have a tiny waist even if the rest of me is still big' just doesnt happen. Theres tons of people on here that can give you better and deeper explanations than me, but i thought i'd reply anyway because i did the whole thousands of sit ups a day thing and you can trust me when i say it doesnt work. Instead, doing 3 sets of 15 sit ups every day is a balanced and healthy way to train your ab wall. In addition think about jogging/running/bike/aerobic classes and maybe later on doing a little weight lifting to help define certain areas of your
body that you would like to (arms for instance).
Hope this helps and please dont get disheartened, losing weight isnt easy no matter who says they have a 'miracle
cure'. It takes a hell of a lot of determination but you can do it!
Regards getting a sixpack, its actually REALLY VERY hard to get one of those. Are you thinking of the Olympic athletes whos
abs we see crunch as they race down the track style 6 pack? Real hard work for that. Theres other posts on here i read just yesterday about getting a 6 pack so if you search 'six pack' i'm sure theres some reading for you to do.
Take care and keep us posted,
Skater.