Its all about practice, feedback from someone knowledgeable and iteration. Open up photoshop and try to create a design. You will bumble around like a fool for the first day - even if all your trying to draw is a smiley face.
But as you do this and as you scour the internet for tutorials and start practicing, it gets easier.
Get feedback from someone in the biz (it takes 10 minutes to look at a design and give feedback). Heed the advice and do more designs.
And all said and done the information to do this is already out there on the internet.
All in all, for me, it was just another case of overcoming my learned helplessness.