I second VirtuosityByDesign's suggestion. The practice will be essential, you'll learn the errors as you go, unless you're fortunate to have a veteran designer by your side/office. Find a community college class, but talk and look at your potential teacher's portfolio to make sure that they actually have their own style and skills to impart.

Additionally, anytime I come across a design that captivates me, I examine it thoroughly and break down 1) why it caught my eye, 2) what's distinctive or new about the design and 3) how was the final image achieved (tools, techniques, talents, etc.)

Good luck and welcome to the field!