One of the best tools ever is Sketch Up and best of all, its free!!! While many practices are sticking with paid programs such as rhino 3d or Autocad more and more are changing to Sketch Up. Im still very much learning, even after years using the application but the power of it can not be underestimated. Bellow is a list of online resources to get you started with the actual program.
There are some great videos for learning different parts on http://www.sketchup.com/training/videos.html#
Once youve learned the basics the Sketchucation is the Mecca! http://sketchucation.com/ if youre willing to pay for it!
If not then there are free resources. The author of Sketch Up for Dummies has posted a few tutorials on their youtube channel- http://www.youtube.com/aidanchopra
I will do another post later for textures, add ons and models later… In the mean time if you have tips please leave them in the comments.