Once you have a good grasp of the fundamentals of design theory (golden ratio, rule of thirds, color theory) it's really easy to adapt to different trends as you see fit to what you're working on. I agree with what some of the other guys suggested here, a quick way is to just copy what other sites are doing instead of trying to invent something yourself.
By copying, i don't mean a verbatim clone of other website's visuals. Look deeper into how a site is laid out, where certain elements are positioned, and try to figure out why the designers visualized the content the way they did.
All this would be assuming that you have a good understanding of photoshop/illustrator. If you don't feel comfortable using photoshop. I'd suggest you to either spend some time to pick it up (sites like tutplus have some basic tutorials that are pretty easy to follow http://psd.tutsplus.com/category/tutorials/interface-tutoria...), or you can just buy a prefab theme from themeforest like pius11 suggested.