In that case, i'd get yourself looking at learning photoshop first (if you have access to photoshop), maybe look at Lynda.com or VTC for paid online training videos. Or YouTube, or Adobe help for free stuff. Alternatively there is cheaper software out there if you haven't got photoshop. I self taught myself photoshop using the adobe help, vtc and lynda.com.
You could look at a part-time course in graphic design at a local college.
I've also just found this which might be helpful https://www.creativelive.com/design/graphic-design?via=topic-list_2
Maybe also look at local courses in art, which will teach you visual skills other than graphics, this will teach you about colour relationships and slihtly alternative ways of thinking.
Another thing worth doing, is taking your wireframes that you've already done, and in your free time, build on those to add visual design. Prototyping software like Axure allows you to do high-fidelity wireframes, I've also used a cheaper software called Flairbuilder, which allows you to do fairly high-fidelity wireframes too. Once you have some images with some visual design pop them on here and ask for a critique.
Do you work with a designer currently who adds to your wireframes?