These resources will be helpful for you as you begin work on your Rube Goldberg machines and your posters. Remember: if you can't find something or you're not sure where to start, please ask!
Sometimes, the best place to start with designing something is seeing other people who did it right. In other words, YouTube!
You'll also need to explain what types of forces your machine uses. The Physics Hypertextbook has a chapter on different types of forces that might be useful as you start to imagine what you could use!
Need more? Ask!
The POA Library has a ton of resources that might be useful for you while you're working on your posters.
If you want more, see the resources on the general Physics guide from the library.