Landscaping -- which in an aquarium is called aquascaping or reefscaping -- is part art and part science. You have to take into account the needs of the species in question while striving for an aesthetically pleasing display. You need to balance utility and beauty when setting up a saltwater tank.
Research and Planning
You increase your chances of success if you take time to research and plan your aquarium before you start. Marine organisms like fish and coral have specific requirements in their physical environments; you must plan around these from the start. For example, some types of corals can sting each other and need to be separated. It's usually easier to accommodate such concerns if you plan for them from the very beginning. Saltwater equipment and organisms often cost quite a bit, so a trial-and-error attitude can result in a substantial waste of money.
Once you have your plan written up, arrange the framework of your tank. Add sand, then base rock. Base rock is hunks of rock, usually low-grade live rock or tufa rock. This forms the "skeleton" of your tank design. Secure the rock by drilling into it and concealing a PVC framework inside of the rock or by affixing the pieces together with aquarium putty. The most common way to arrange rock in a marine aquarium is to set up a sort of rock wall, tallest toward the back of the aquarium, sloping slightly toward the front. However, this old standard is starting to give way to other designs, like arches and "islands" of live rock instead of a single continuous wall.
Position your equipment with the needs of your organisms in mind. If a coral needs strong currents, you may need to position it near filter outlets. At the same time, you may also want to hide equipment behind your rock. There may be some back-and-forth between positioning equipment and positioning your base rock.
Once the equipment and the rock are set up, start to add sessile, or unmoving, invertebrates. Position them based on research. Some inverts prefer strong current. Place these near pump outlets. Inverts also have substrate preferences. For example, some anemones prefer to anchor themselves in sand; others prefer to latch onto rocks. Also, some inverts need strong lighting. Place them near the water's surface. Others need dim lighting to thrive. Place them under overhangs or at the very bottom of deep tanks.