You need to provide more information about what tools you are currently using.

For the purpose of creating wireframes, I would suggest using Balsamiq, and avoid doing any styling at all. This is good for the phase when you are working out screen flow and layout.

If you are doing a mockup, i.e. something that can look like a screenshot and will determine the graphic style of the app, then you really need to work with a graphic designer. Would you ask a non-programmer to write code? It's the same with graphics -- there's no substitute for a professional. At the stage you're at, you should be able to find an independent freelancing designer quite inexpensively in any decently sized city.