Here's a quick video tutorial for placing a prefab:
Setting a prefab, or mesh, is the easiest way to set up content that you have purchased on the marketplace (You can access the Marketplace through the cell phone tool). When you click on the prefab button, a new menu opens:
You can either select a prefab you have used before, or if you click the folder, go into your library and choose items you have uploaded or purchased on the marketplace.
Once you have selected the item, it will appear inworld at the end of your cursor. Once you click on the ground it will then be placed in the space. Before you place it you can rotate the object by right clicking with the mouse. After you have placed it you can move, duplicate and delete the item using the tool panel; you have to select the item first by clicking the select button (the arrow) and then using the tools If you right click a placed item and choose “edit the item”, you can customize it more using the advanced building tools. (Note: Advanced building tools are only available to verified accounts) There is much more to learn about building, but these basics will get you started. The wiki has much more information, and the forums are full of helpful people if you want to ask questions.