By default, when you create a slave site, you have specific content (menu, sections, categories, articles, forum, ...). Only the PHP code (the files and folders) are shared between the websites.
You can only share some extension content (DB tables) between websites.
If you are on unix platform with Symbolic Link allowed, then to create websites that share the same files (including the templates directory), you have to create a JMS template rule that declare the "files and folder" tabs with Symbolic Link action for all the folders and "Special Copy" for the images and "templates" folder.
See tutorial video 0.b slide 34 and CP-05
www.jms2win.com/tutorial#jms12x-0b
www.jms2win.com/tutorial#jms12x-CP-05
Concerning the modules, you will have to re-install them in each slave sites.
If you create a "pre-configured" slave site with all the module installed, this can help you create the other slave site faster with the pre-installed modules.