Most people in the gym when starting off have a goal to lose weight. Once they achieve this they then aim to build muscle. So whats the difference between losing body fat and building muscle? The cell structures first of all are completely different between a fat cell and a muscle cell. Its easier to break down and make use of a fat cell than it is with a muscle cell. So in terms of burning fat and building muscle, think of it like this

- If you're building a wall it could take 12 hours, however if you want to break it down, it can be done in minutes. 
Not that losing fat can be done in minutes however the logic is it takes a lot longer for the body to build muscle than to lose fat.

There are however ways each process can be speeded up to some degree. A more dramatic calorie deficit will cause the body to lose fat, however a by product of this is that the body will also turn to muscle tissue as it is more nutrient dense than body fat (if protein intake is kept high, you can somewhat delay this). 
To build muscle the body really needs a good amount of proteins, carbohydrates and fat.
 If your looking to start out in the gym, and your primary goal is to build muscle the best advice we could give is to be patient. It will take time and you will have to increase calories at some point but don't do it too soon, the main objective should be to increase Lean Body Mass rather than body fat.