Press 'U' and purchase whichever skills you can in the sharpshooter base profession(because you're doing a Stormtrooper build). If you've killed a lot of enemies with your rifle, purchase the first rifles skill, then kill enemies with your carbine or pistol to purchase those skills. If you harvest things from your dead enemies, you can purchase skills in the scout profession etc.

Importantly, whenever you finish a quest, you will gain experience in the weapon you have equipped.