The beauty of LEGO is truly revealed when you start building without instructions. It is best to start with a large collection of standard LEGO pieces (i.e. rectangles, squares, etc.) and an assortment of speciality pieces, perhaps acquired from demolished projects that used a box kit. A large pool of standard pieces will allow you to use the colors you want and still have enough pieces, and the speciality pieces add the occasional design function you will need to express your creative ideas.
The design process usually starts with an idea. Visualize what you want to build. Start with the basics. If you want to build a house, first design the framework. After you have a frame, add a roof, and then exterior decorations.