When you have the 'Auto Shoot' enabled your weapon will automatically reload when your clip is empty, using this feature though will mean you will not be able to use your scope.
If you have 'Auto Shoot' disabled the reload and scope buttons are above your shoot button which is located at the bottom right of the screen. Make sure when you play with the 'Auto Shoot' disabled that you reload wisely because you will not be able to shoot again until the reloading animation has completed. To use the scope tap the the scope button, when using the scope you will significantly increase your accuracy as you will be able to zoom in for a clearer view to shoot enemies with your base gun.