The first thing you need to do when creating a sketchup page is download sketchup. This can be done with the link above. After doing so you may want to view some of the tutorials in the program. This will help you learn more about the program and help you get started creating your sketchup scene.
Now after downloading and installing sketchup you should become oriented with the tools that you will be using. At the top of your screen you will see a tool bar. This is where you will find most of the tools you will need to use. Click on the different tools and mess with them to see what they do. Try creating different shapes and objects and color them. If you can't find a tool you want to use try looking in the tools tab on the menu bar.
You should now be familiar with the various tools in the program and have some knowledge as to what you can make and do with SketchUp. Now all you have to do is use your imagination and create whatever you want. If your having trouble with the program or struggling with anything in SketchUp you can always look it up in the Help menu or go back and view the tutorials.