The first 20mins or so of doing any form of exercise, you will be utilizing the energy from glycogen stores and cards in your body.
You will only tap into your fat stores AFTER that period of time for energy; therefore fat burning normally occurs only 20mins into exercise..

For weight loss and at the same time pack on some muscle, i suggest working out first for about 20mins, then doing a 30mins cardio session or more.
And also, always remember to allow your body to get sufficient rest.. too much stress on it will hamper all gains/losses(fats).
Gluck!