JonST sit ups are probably one of the worst exercises going. Your time would be better spent doing crunches or a core exercise like squats. Less risk of injury, better exercise for the body. I hate sit ups!
To raining supplements will make around a 1% difference if your diet isn't right. If your diet is good, they'll still only make around a 10% difference. They're expensive too, unless you want to body build I'd forget about supplements. A diet low in fat and high in nutrients is your best bet.
By the sounds of it you're over thinking this. If going to the gym seems over complicated and tedious the less likely you're going to want to stick to whatever plans you make. I'm guessing your already joined to a gym if you have an instructor. If your membership includes classes, try some of them! Talk with the staff and ask which would be the best for your level of fitness. They will be ran by an instructor who'll go through everything with you. Most classes are between 30 mins to an hour and in that time you'll get a balanced work out.
If you don't like the idea of classes. Building up stamina doesn't have to be complicated. Stick some good music on your mp3 and just do some cardio. Running machine, Cross Trainer and Rowing machine to mix it up.