First of you need to craft a weapon. Most of the MCPE weapons are crafted with redstone, iron ingots and gunpowder
and the specific ammo based on what weapon you want to craft.
Different weapons use different user interfaces. For example, with one button on each side of the screen you can choose whether to shoot or to aim. Exactly how each MCPE weapons works is quite self-explanatory in-game and that’s one of the clear advantages with the mod for MCPE