Figure out how many calories you need to consume to lose weight at a rate that you think is good (I'd recommend 1-2lbs per week, otherwise it might be too quickly and you lose muscle, or find it hard to maintain, or it will be too slow and it can be hard to see the progress and you will never get there). Then, measure those calories every day (just get used to using a scale for many foods and following all the labels etc). Stick to your diet and your calorie log. If you struggle and have to go over, go over just by a minimal amount (like 100-200 calories).
Get in a routine. Just find what works for you and eat basically the same things/do the same kinds of workouts. When you notice progress (like lose a few pounds), give yourself a reward like a day where you don't count calories, have a meal out, or whatever. Then get back on track as before. Weigh yourself no more than once per week, and keep a weight log so you can see your progress over time.
I'll give an example of myself. Lately I've been consuming about 2200-2300 per day, which allows me to lose about 2lbs per week with minimal exercise. I talked to my doctor, and we figure I could afford to lose about 15-20 lbs over the next 6 months or so. At that point I should be pretty lean. I told him I think I can achieve that without issue.
You can exercise or not, it's up to you, although I'd recommend doing some (though preferably quite a bit if you really want to lose a lot of weight, especially if you don't want to lose muscle; it is worth it to maintain or build muscle while losing fat). The reason I say it's not necessary is because some people literally can't exercise, for one reason or another, and you can indeed lose weight from diet alone (this is the essential factor). Right now I'm in recovery from an injury, so I can't do much in the way of exercise, but I can still lose weight via diet.
Remember that the medications can make it harder for you, but they don't force you to gain or keep weight. They can change/slow your metabolism, so you will absorb more calories, and they can increase appetite, so you'll eat more. But unless there is something wrong with the meds (in which case you need to change), they aren't actually forcing you to be of a certain weight. Diet and exercise can counter what they're doing.