As others have said, start small, build a house or a small diner. Once you have your basic shape try adding details and features to it, you may find you mod and rebuild this one creation a dozen times before your happy with it or before it resembles your vision. Just build the facade to start with not the whole building ( whole buildings are brick heavy and can easily become too much early in the design process).
Try something cool like this,
Don't limit yourself to the town forum for inspiration and building techniques, you'll find all sorts of nifty techs in castle, sci fi, and the pirate MOC forums, that will come in handy when tackling fine detail.
Once you start MOCing you won't be so worried about mixing your sets as they're all just bricks at the end of the day.
Most importantly don't be afraid to post your progress, while some may run you down for your efforts, (those comments are not in the spirit of EB, and are frowned upon by most of us) most will give you feedback to help you improve your builds and encourage you to keep going.
We all started somewhere and people learn at their own pace and have their own style, if everybody built the same we'd get tired of looking at each others creations.
Good luck and stick with it.