First, make sure you install the Dropbox desktop application. It creates a new folder on your computer called Dropbox (or two separate folders if you're a Dropbox for Business user and have connected your personal and work accounts). Your Dropbox folder works just like any other folder on your hard drive, except everything in your Dropbox folder automatically syncs to the web and to any other computer with Dropbox installed.
It's easy to add files to Dropbox. Move your files into your Dropbox by dragging and dropping them into your Dropbox folder. That's it. The files in your Dropbox folder will automatically be synced online and to your other computers. You don't have to do anything. You can even use the Save As... menu option in most applications to save files directly to your Dropbox folder.
After installation on Windows, your new Dropbox folder will be located inside your home folder by default.
The window will disappear, your files will start uploading, and you'll be shown a progress indicator. That's it, you've added files to your Dropbox!
Dropbox for Business users: If you have connected a personal and a work Dropbox, make sure to select the correct Dropbox from the sidebar before uploading the file.
Wanna see something awesome? If you use Chrome, Firefox, or Safari, you can upload files by dragging them from your desktop onto the Dropbox website.
See a short video on how to add files to your Dropbox, both on the website and using your desktop application.
Dropbox is better than a network drive. Files in your Dropbox stay on your computer's hard drive, so you can always access your files even if you are offline. Dropbox works in the background and will automatically sync any new changes the next time you are online.