You got a USB to microUSB cable that came with your N7. The cable can be used to charge your N7 or transfer data to your PC.
To transfer data:
- turn your N7 ON and plug the USB end into your computer and the microUSB end into your N7
- a window should pop up on your PC saying a new USB device has been connected - select the show directory or open an instance of windows explorer
[ you may need to set the default USB device type Settings > Storage > 3 periods symbol at top right > USB computer connection > Media Device (MTP) checked ]
- you should see a Nexus7 'drive' device with a folder called 'internal storage' when you select this, the full directory structure of the N7 will be visible
- it may be a good idea to transfer files to the default folders eg. \Movies \Books \Music \Pictures etc to start with and you can make new folders on your N7 within those later
Drag and Drop or Copy and Paste will work fine on the PC
On the N7, get yourself a decent free File Management app like ES File Explorer, X-Plore, File Expert, Astro File Manager, others
You can also use AirDroid to control the whole process from a browser screen as repligation mentions if you want an alternative.