Sketchup is 3D modeling software. It lets you create your own 3D designs.
You can use the web version of SketchUp for free here:
Note that if you want to save any of your creations, you have to create a "Trimble" ID. I would suggest that you click the "Save" link at the top left before you start working and get this step out of the way first.
When you are ready to print your creation, click on the folder icon at the top left, choose "download" from the menu and then choose ".stl file." This will download a file that you can then use with a 3D printer.
Take a look at the SketchUp tutorials here:
The web version of SketchUp has fewer features than the desktop version that they use in these tutorials, so just note that it's going to look a little different as you follow along.
You can print your stl files to our 3D printer with the help of a teacher or Mrs. Russell. Note that the size of the build plate on our printer is 4" L X 5" W X 5" H (length, width, height) which is kind of small, so keep this in mind when you are designing. You can print one color at a time. We have 11 different colors of filament to choose from: White, off-white, light gray, dark gray, black, yellow, red, green, blue, purple and orange.
Here's more information about the printer we have: https://www.makerbot.com/replicator-mini/