To make an object transparent, you need to edit the materials that are used on the faces of the object. to do this, open the materials window by selecting the paint bucket tool. Then you can use the menu to navigate to the appropriate material or use the eyedropper tool to select the material. Once you have the material selected, you should be able to use the opacity slider to increase or decrease the opacity accordingly.
If you need any help with these steps or run in to any problems let me know so I can try and find a solution for you