First off, fat does not 'turn in to muscle'. So, you are best to focus on one goal and then the other to maximize your results.
If you are trying to burn fat first, change your diet so you are in a caloric deficit of about 500-1000 Kcal/day. This includes the energy burned during workouts so, on these days you will have to consume more calories (probably in the form of protein).
Taking a fat-burner only serves to expedite the process by increasing the number of calories burned in a day. This may be beneficial to you, or it may not. Remenber, these are ergogenic aids, not miracle pills! They can help you reach your goals faster, but it is incumbent upon you to make the appropriate dietary changes!
Lastly, try not to get too caught up in the details. You have to get close to your desired caloric deficit every day, but that's it. This is not a perfect science, and we are not splitting the atom here. You just have to make sure that you are in a 500-1000Kcal deficit every 24hour period, through a combo of diet, exercise, and proper supplementation. If you maintain this, you will undoubtedly lose weight. Once you have reached a good bodyfat %, you can bring your calories up a bit and focus on building more mass. THis isn't to say that you won't build any mass while losing fat, it's just that your gains won't be as dramatic while in a calorie restricted state. Keep it up over the long term and you will succeed!