Warm up - 10 minutes on the rower at about 32 pulls per minute/ten minutes on a recumbent cycle.
Actual exercise - use the weights machines. Follow the instructions, make sure you are correctly positioned, don't go mad and don't do hundreds of reps on something you can't even feel, go for something you can just about do 8-10 reps of slowly and controlled before stopping, drinking some water and then doing another set, repeat. If you've overestimated what you can do, drop the weight down one pin or reduce reps by 2 until you can complete three sets.
Spent around 30 minutes moving between machines. You're too heavy right now for squats to be safe for your knees (I know this from experience), so make sure you include the Leg Press.
Warm down with another ten minutes on Rower or Recumbent.
Depending upon how you feel each visit, you might want to spend longer on the warm up/cardio element, you might need more leg work (or back, or arms). Listen to what your body is telling you.
Sorted. Do that around 3 times a week, chuck in an easy walk twice a week (or walk back from the gym if you can) of about a mile/20 mins and you'll start seeing a difference very quickly. Adding some gentle stretching or foam rollering can help recovery times. Drink loads of water, eat well, make sure you always have at least 24 hours between doing weights.
If you can, make at least one of those three sessions in a pool instead, so Aqua Fit or just swimming, as it will feel really good without causing impact injuries as a dry land class can do.
Classes are great for those who love them, but I know from direct experience that at size 24, you're dropping an awful lot of weight onto your feet, knees and hips before your muscles are strong enough to cope with it. And the pain of overdoing it because somebody's there shouting at you to put some more effort it, as this is fun, and you're aware that other people seem to be finding it easy is like nothing else.