I just started building one myself with our school kids. We're going to build a bigger one after that. I started by using the OpenAg github information found here: https://github.com/OpenAgInitiative/gro-hardware
This has a great list of materials with links to purchase the items (the file is labelled BOM for bill of materials) and pretty detailed instructions (not perfect, but really good). You can use the directions as is, or as a starting point and diverge from there (e.g. make it bigger, smaller, add different sensors, etc.).
You'll need CAD software to open/view the model files. If you don't have one, you can use the eDrawings free software here: http://www.edrawingsviewer.com/.
Hope that helps!