If the ball is approaching your player in the air and you want them to volley the ball rather than head it, you can hold down the LT/L2 button and press the shoot button and your player will attempt to volley it. This will include techniques such as bicycle kicks and half volleys so can result in spectacular goals!
At the time of writing, which is at the release of the game, volleying is overpowered and will probably get nerfed at some point but this is currently a great way to regularly score goals and these types of goals look amazing when they go in!