Hi mate,
Well imo you have to pick a goal, fat loss or muscle gain. Im guessing you want to lose the fat. You really need to work out how many calories you are taking in each day. Be strict and honest and include everything ! One way is to lower your calories, this is a pretty regular diet. However it probably isnt the best for the bodybuilder because aswell as losing fat you also lose msucle.
There are some brilliant write ups in the diet section of these forums on keto diets, timed carbs and carb cycling. If you have never herd of any of these dont worry and sont be put off. They are pretty simple once you give the posts a good read make a lot of sence. You will learn alot.
Outside diet, uphill walking and cardio in teh mornings will help. Also just be as active as possible through the day , walk to the shops, always take the stairs, and of coursecut out all your **** food, drinks etc.
Also personally Id say sits ups arent really going to help you lose fat, they wont hurt but you wont be spot reducing fat on your belly. Persoanlly Id say youd make it look worse as your abs are gonna swell up. SHort term that is. A set of squats are going to burn ALOT more calories, realse more GH which should both help burn off that fat
Hope this helps!