I was in a similar situation about 6 months ago, about 10kgs over where I wanted to be. Low carb diets are all the rage right now, but aren't much fun and it is easy to put the weight back on when you go back to normal eating.
What I did was to eat a balanced healthy diet (plus junk food and chocolate 1 night per week to treat myself) plus 4-5 40min cardio sessions per week, and a few weight sessions to keep muscle tone. My bodyfat has gone from 20% to around 12% in this time.
Once you get your fitness levels up a bit, consider High Intensity Interval Training. You can get a great cardio workout in 20 mins. That's what I do now, combined with 30 mins weights a few times/week (maintenance), and I am still losing bodyfat without worrying about what I eat or living in the gym.
If you like tuna, make a sandwich with some lettuce, low fat cheese and a few other fillings. Snack on fruit and yoghurt, I also sometimes have weetbix or le rice. I would avoid supplements and stick with real foods if possible.