Designing in 3D does take some getting use to but if you are going to use Sketch Up here are some great tutorial sites:

http://designerhacks.com/sketchup-tutorials/
http://www.sketchup.com/learn/videos/58
https://www.youtube.com/user/4sketchupgo2school/videos

If you are just looking for designs then I would recommend thingiverse.com its a great place to go and find just about anything.

Hope this help.