Losing Weight Methods

So my stepdad was just diagnosed with Type 2 Diabetes, and we made a promise that we'd lose weight together (for his health and my self-confidence). I'm about 5'5" and 190-200 lbs, and I don't have a specific weight loss goal yet. Though, by the time prom rolls around, I'd like to be down to a size 12/14 (from a current size 18). But, my main concern is my health and confidence rather than the numbers.

Are there any specific dieting methods, or even quick exercise methods that have helped you? I'm looking for some recommendations. Thank you!